Most of the time, when you hear about a transaction, you think “yeah, that makes sense.” Free-agent contract terms are seldom surprising. And trades between teams are also seldom surprising, in terms of how much each side seems to be worth. Every so often, though, a move will give you pause. There was a trade between the Red Sox and the Mariners, Monday, and it arrived a little differently than usual. It didn’t make immediate sense for both sides.

After having lost out on Hisashi Iwakuma, the Mariners turned around and picked up Wade Miley, along with Jonathan Aro. They wanted a starter, and they recognized that the Red Sox had some excess depth. Nothing surprising there. Nor was it surprising that the Mariners were willing to include Roenis Elias. The part that engaged the eyebrows was the final piece, that being Carson Smith. What this is, today, is the Wade Miley trade. What this very well might end up looking like is the Carson Smith trade. The Red Sox did well to add a reliever with many years left.

From time to time, you’ll see a move and you’ll think it favors one side. Some moves do favor one side. You can express yourself and stop there, or you can try to think about why the one team might’ve been a willing participant, given that every team has so much information. What it looks like is that either the Mariners are quite fond of Miley, or they’re a little down on Smith. It’s smart to at least try to understand from both perspectives.

Let’s start from the Red Sox perspective. It’s the easier one. By adding David Price, the Red Sox wound up with too many starters, and that’s not a bad situation to be in when the market is going so wild over starters. There’s been talk about Miley, Clay Buchholz, and Joe Kelly. Miley’s the guy to go, at least at this point. He’s under guaranteed contract another two years, with a third-year option.

Miley obviously isn’t nothing, as a 29-year-old who’s surpassed 200 innings. Over two years, he’ll get paid right around $15 million; over three, he’d get right around $27 million. That’s not a bad rate for a decent starter. But now let’s consider a point of reference: J.A. Happ. Happ recently signed for three years and $36 million, and, like Miley, he’s left-handed. Over the past three years, they have an almost identical ERA-, and an almost identical FIP-. On the one hand, Happ is older; on the other hand, Happ has been excellent more recently. They’re comparable pitchers, so while Miley will be underpaid relative to what he could get on the market, it’s not a dramatic difference. He’s a decent asset.

Miley, to be sure, is better than Elias. Elias is functional starter depth, and maybe one day he’s in the bullpen. Elias is a small-time piece, in the way that Aro is a small-time piece. Smith is the get. Smith is the kind of guy Dave Dombrowski could’ve gone out to get, if he didn’t want to pay the premium price for someone more established. Smith is a guy some teams might’ve considered as a quieter alternative to Craig Kimbrel. Dombrowski just went ahead and got them both.

The comparison I have stuck in my head is between Smith and Ken Giles. They’re both young, with some ninth-inning experience. They both have five remaining years of team control. They’ve both struck out a third of opposing hitters. They’ve both allowed a similarly-low batting line. Giles throws a harder fastball, but on performance alone, they’re extremely similar. Teams have called the Phillies on Giles, and they’ve talked about upper-class prospects. The Phillies were said to have asked the Astros about Lance McCullers and Vincent Velasquez. The Phillies want a haul for Giles. The Red Sox got Smith for Wade Miley.

In case you don’t know how good Smith just was: he was fifth among relievers in WAR. He was simultaneously a strikeout machine and a groundball machine, so he didn’t get hit, not even by lefties. This is an offseason in which it looks like good relievers will be heavily rewarded, so it feels like it should’ve taken more to pry Smith away. Especially since the Mariners think they’ll contend, themselves, and now their bullpen is far more thin. They need a Carson Smith. They needed another Carson Smith when they still had Carson Smith.

From the Red Sox side: they dealt a cost-controlled starting pitcher who can look like a No. 3. They added a young reliever who, statistically, compares to many of the best relievers in the game. If you believe in Smith’s numbers, you like this trade for Boston. You have to. Smith could have long-term closer potential. He’s already been excellent.

Flip it around, now, and look at this from the other side. The Mariners dealt a starter and a reliever for a starter and a reliever. Miley is fine, and he’s looked better than that before. He’s fairly young and he’s been durable, and it’s good to have a proven pitcher locked up for up to three years at a modest price. Miley would get more than Happ on the market. Maybe he’d get something like the Iwakuma deal, or even the pending Mike Leake deal. It would’ve cost the Mariners more money to plug the hole with a free agent.

They shouldn’t miss Elias too much. His 2015 was worse than his 2014, and his 2014 wasn’t all that good. He’s probably a sixth or seventh starter. Some of the focus ought to be on Aro. Much of it, of course, remains on Smith.

I don’t want to build Aro into something he’s not, but he just debuted last season. He didn’t pitch much in the majors, but in Triple-A, he did throw a ton of strikes while missing an above-average number of bats. He’s not a copy of Smith or anything, but he’s a guy whose stock is rising, and he should be able to help in the big leagues. He’s not a pointless throw-in.

And then there’s Smith. As I’ve thought about this trade, I’ve thought about the potential difference between what Smith’s been, and what he could become. I have to figure Jerry Dipoto doesn’t quite buy Smith’s numbers as indicators of his ability going forward. I personally like Smith a lot, but I could understand why someone might suggest we’ve already seen him at his best.

Simply understood, Smith is a sinker-slider righty reliever. There are plenty of those, and while there are few of those with Smith’s numbers, he doesn’t throw with eye-popping velocity. As a sinker-slider righty, he’s someone from whom you’d expect a pretty big platoon split. It hasn’t materialized yet, but that doesn’t mean it won’t. There were times Smith was protected from facing many lefties, and that wouldn’t be the case if he became a regular high-leverage reliever. If lefties became a problem, then Smith would look an awful lot less like a ninth-inning option.

Then there’s the matter of Smith throwing sliders with almost half his pitches. In any projection, you have to try to account for individual injury risk, and though doing that well is obviously a challenge, Smith might well be at greater risk. Dipoto might not love Smith’s odds of remaining healthy, and it’s worth noting he had a small velocity decline toward the end of last season. Definitely might’ve been nothing. And maybe his arm is up to the challenge of pitching, as much as anyone else’s is. I’m just trying to present what might be the argument.

As I think about it, I still like this more for the Red Sox. I don’t love Miley’s command, so I don’t love his chances of taking a step forward. As a mid-rotation pitcher, he’s boring, and only modestly valuable. Smith has already resembled an elite reliever, and he’ll cost nothing for years. We don’t understand reliever projections and injuries nearly well enough to say that Smith has peaked. I like what Boston did. I just see why this is also what Seattle did. It’s not insane, and it’s not unjustifiable. It’s just questionable. Unusually so, these days.