Thats largely because he pitched in a pitchers park with the best defense in the NL though. His K/9, BB/9 GB%, HR/FB etc has all stayed pretty much steady throughout his career. The only thing that has really jumped around are his BABIP, LOB% and ERA which are at least partially out of his control. I cannot stress enough that ERA is a faulty stat for judging pitchers, its a team stat. A 4.00 ERA pitcher for the White Sox is much better than a 4.00 ERA pitcher for STL, hitters park, AL vs NL, harder competition, defense isn't as good. You really need to dig deeper than just ERA when looking at a pitcher who has moved from one team to another.