(Stats used are prior to his July 21 start)
Based on his strikeout rate, unintentional walk rate, and the types of batted balls he's allowed, Peavy should be doing well. Basically, he's allowed 3 more singles, 7 more doubles, 3 more triples and 3 more home runs than you'd expect given his batted balls allowed and an average fielding team in a neutral park. Now, the three home runs could be explained away by the fact that the park has been kind to home run hitters this year. The extra hits that stayed in the park are strange given that the Padres turn batted balls into outs better than any team in the Majors other than the Tigers.
Combine that with the fact that he's grouped those hits, walks and outs in a most unfortunate way and you're looking at an ERA not in line with how Jake has pitched.
There's a few ways one could disagree with this.
1) You believe that a line drive allowed by one pitcher is more/less likely to be an out/hit than one allowed by another pitcher. Read: it's a pitcher's skill that determines if a ball lined to center is caught or not.
2) You believe that batted ball data is not tracked well enough to be used in any real analysis. Debatable.
3) You believe that component ERA means less in determining how well a pitcher pitched than actual ERA. Read: Pitcher A and Pitcher B both allow three singles and a double, but the fact that Pitcher B allowed them Double, Single, Single, Single instead of Single, Single, Single, Double means he pitched better than Pitcher A.
4) You really believe Peavy sucks and there's no convincing you otherwise.