The first month of the season is always littered with surprises.

Take the Chicago White Sox, at 9-9 they are a game removed from first place in the American League Central. More surprising than the White Sox's record is this: Despite trading Chris Sale, their pitching staff leads the majors in ERA. Yes, it's early. Yes, it's unlikely to continue. Still, the White Sox have posted a 2.91 ERA -- a fair shake better than the second-place New York Yankees, who check in at 3.17.

The main reasons for the White Sox's shiny mark? Their bullpen and defense.

The White Sox's rotation has been good -- Miguel Gonzalez was an impossibly savvy signing last spring -- but Chicago's bullpen, which trails only the Yankees in ERA, has been even better. The White Sox entered Tuesday with eight relievers who had appeared at least once. Among those eight, just two had an ERA above 1.50. Everyone knows how fickle ERA is -- especially for relief pitchers and especially in small samples -- so it's impressive in a sense that the White Sox's bullpen lacks a crooked number.

Closer David Robertson seems unbothered by trade rumors, as he has struck out 12 of the 25 batters he has faced. Tommy Kahnle, a flamethrower whose name might sound familiar to Rule 5 wonks, has done Robertson one better: He has fanned 14 of 26 batters, all while issuing just one walk. Zach Putnam, who went on the disabled list Tuesday, has an impressive strikeout-to-walk ratio, too, having struck out nine and walked one. Ditto for Anthony Swarzak. And so on.

Reliever IP SO/9 ERA Michael Ynoa 9.1 6.75 3.86 Zach Putnam 8.2 9.35 1.04 Nate Jones 8.2 10.38 3.12 Anthony Swarzak 8.1 9.72 0 Dan Jennings 7 6.43 1.29 David Robertson 6.2 16.2 1.35 Tommy Kahnle 6.2 18.9 1.35

The focus in the preceding paragraph was on the aspects the relievers exert the most control over. Why? Because it's irresponsible to write about the White Sox's ERA without noting how well their defense has played. Chicago completed play Monday having converted a majors-leading 74.8 percent of balls in play into outs. How good is that? Consider that the 2016 Chicago Cubs, possibly the best defense of all-time, turned 74.5 percent of balls in play into outs.

Credit some strong middle-infield play for the White Sox's defensive efficiency if you like, but this is the part where it's important to note the difference between descriptive and predictive. No one is saying (or should be saying, anyway) the White Sox's defense and pitching staff will continue to produce like this. Rather, this is simply a look at what has happened to date.

And so far, the White Sox have been pitching and defending their way to impressive marks.