According to the best data I can access, so far this year there have been 425 pitches thrown at least 100 miles per hour. Andrew Cashner‘s got one. Nathan Eovaldi‘s got one. Matt Harvey‘s got two. Bruce Rondon‘s got 104. Aroldis Chapman‘s got 196. To set a cutoff at 100 is arbitrary, but it feels natural, and 100 definitely has the feel of a magic number. A fastball at 100 is, officially, a fastball in the triple digits. Within the realm of 100+ mile-per-hour fastballs, Rondon and Chapman, combined, have thrown more than twice as many as everybody else. The next-highest total after Rondon’s 104 is Kelvin Herrera’s 45, and there’s long been talk that the PITCHf/x in Kansas City is miscalibrated.

What Chapman’s got over Rondon is peak velocity — Chapman, this year, has topped out at 104. What Rondon’s got over Chapman is consistency — 24% of Rondon’s pitches have reached 100, against Chapman’s 19%. Rondon has the harder average fastball. If you isolate only those fastballs thrown at least 100, Rondon and Chapman tie with an average velocity of 101. Clearly, these are the game’s premier flame-throwers. But while they both throw similar heat — similar, virtually unparalleled heat — the results have been considerably different. We’ve been seeing two types of 100 mile-per-hour fastballs.

Take a look at Chapman’s fastest swinging strike:

An unhittable 103.4. Take a look at Rondon’s fastest swinging strike:

A seemingly unhittable 102.8. What Rondon lacked in actual velocity, he made up for in perceived velocity, as the hitter sees inside pitches as being faster than outside pitches. Watch these clips and you’d think, yeah, these are both swing-and-miss fastballs. Two-strike fastballs, putaway fastballs. Fastballs that can be both the foundation and the structure above.

Yet the numbers tell a different story. We’ve long known about Rondon’s heat, and it was because of that heat he was in the mix as a spring-training closer candidate, but the overall season numbers might tip you off. As an American League reliever, Rondon has so far posted a league-average strikeout rate. As a National League reliever, Chapman has so far posted double the league-average strikeout rate. The similarities end with the radar gun.

Baseball Prospectus provides handy PITCHf/x leaderboards. I looked at all relievers who have generated at least 100 swings this year against their four-seam fastballs. That gave me a pool of 143. Chapman’s contact rate allowed is the third-lowest, out of first by half of one percentage point. Rondon’s contact rate allowed is the 94th-lowest, or the 50th-highest. Rondon is right there with guys like Tommy Hunter and Henry Rodriguez, but he’s also right there with guys like Josh Roenicke and George Kontos. You’d expect better from a guy who has topped out at 103.

That’s looking at all fastballs. Let’s narrow down to only those fastballs at or beyond 100 miles per hour. After all, that’s the real purpose here, and as a reminder, Rondon has thrown 104 such fastballs, while Chapman has thrown 196. They’ve posted similar strike rates, and similar swing rates. Batters have swung at Rondon’s best heaters 53 times, making contact 43 times. Of 27 balls in play, they’ve recorded eight hits. Batters have swung at Chapman’s best heaters 102 times, making contact 55 times. Of 17 balls in play, they’ve recorded three hits. A summary:

Rondon, 100+: 81% contact

81% contact Rondon, 100+: 51% in-play

Chapman, 100+: 54% contact

54% contact Chapman, 100+: 17% in-play

The differences between the raw numbers are dramatic. Now there are a few things we have to consider. One is that pitches don’t exist individually in isolation, existing rather as functions of the entire repertoire. In addition to his fastball, Chapman also throws a wipeout slider, that has Rondon’s secondary stuff beat. Chapman’s fastball wouldn’t work as well were it not for his slider. His slider wouldn’t work as well were it not for his fastball. Everything is inter-dependent, because baseball is obnoxiously complicated.

Additionally, Chapman uses his fastball more often in putaway counts, counts where he might specifically be going for a swing and miss. With two strikes, five of six Chapman pitches are heaters. With two strikes, half of Rondon’s pitches are heaters. Of course, this might be due to Chapman’s fastball being so unhittable, not the other way around. Good luck with proving causation.

I think the biggest factor is probably pitch location. Let’s look at all of Chapman and Rondon’s 100+ mile-per-hour fastballs:

Now let’s look only at those 100+ mile-per-hour fastballs that were swung at:

Rondon doesn’t pound the bottom of the zone by any means, but Chapman clearly likes to live up. Chapman’s average fast fastball has come in 2.9 feet above the ground. The average whiff has been at a pitch 3.0 feet above the ground. Rondon’s average fast fastball has come in 2.3 feet above the ground. The average whiff has been at a pitch 2.5 feet above the ground. These are differences of about a half-foot, and while that might not seem very big, the actual pitching zone isn’t very big. Lower fastballs tend to go for contact and grounders. Higher fastballs tend to go for whiffs and fly balls. Chapman uses his heat in a way to maximize swings and misses. He just tries to blow it right by hitters. Rondon is presumably capable of that, but he seems to be going more for strikes and weaker contact. Rondon has tried to stay more down in the zone, whereas Chapman has dared hitters to try to take him deep.

I can’t really speak to their deceptiveness, nor can I speak to their release points. It might be a little harder to see the ball against Chapman. He’s a little taller, so he should release the ball a little closer to the plate. That means less time for the ball in the air, which means increased perceived velocity, which means better results for the pitcher. There’s a lot that might be going on here, but it’s plainly obvious that Chapman and Rondon use their fastballs differently, causing similarly extreme heaters to generate very different results. Chapman tries to miss bats. If Rondon does, he generally does it with a different pitch.

A pitcher is blessed if he’s able to throw really hard. Because of his arm strength, Bruce Rondon could have one hell of a future. Because of his arm strength, he also draws some comparisons to Aroldis Chapman, but there’s a hell of a lot more that goes into pitching than how hard you can throw. Rondon has it in him to develop into a strikeout fireman. If he changes the way he uses his heater, he could conceivably get a lot more swings and misses. But right now it just isn’t the same pitch that Chapman throws. Rondon has a hard fastball, but he doesn’t have a strikeout fastball. Too often, people think those go hand in hand. Everything is less simple than you want it to be.