So, uhh, what’s left? He’s not preventing hits. He’s not preventing home runs. He’s not stranding runners. How is Mark Buehrle keeping so many runs off the board?
The answer might be that he’s not, and a real part of the explanation for Buehrle’s gap between his ERA and his FIP/xFIP is actually a bias in how ERA is calculated. As you know, Buehrle is a ground-ball pitcher, and pitchers who put their infielders to work see a larger share of their balls in play result in errors. Errors result in unearned runs, and unearned runs don’t count against a pitcher’s ERA.
In fact, if we look at Buehrle’s career, 10.1% of all the runs Buehrle has allowed have been labeled as unearned. For starting pitchers since 2002 with 1,000+ innings pitched, that’s the ninth highest ratio of unearned runs in baseball. Some of the pitchers ahead of him include Brandon Webb, Felix Hernandez, and Derek Lowe, which illustrates the point of how ground-ball pitchers tend to have ERAs that are driven down because many of the runs they actually do allow are counted as unearned.
...This doesn’t invalidate Buehrle as a good pitcher, as his durability and consistency are still intact, and even if you judge him by something like FIP-, he still comes out as one of the better pitchers in the league. However, it’s at least worth noting that ERA is overstating his ability to keep runs off the board, and if a team is going to pay for Buehrle’s services, they should adjust their expectations of value down slightly from what ERA says he is worth.