2011 was his best year from a fantasy standpoint, but from 2010 to 2011 he K rate per batter dropped slightly, he gave up 12 home runs in 2010, then 26 in 2011, yet his ERA dropped. Last year his walk total rose to over 80 and his K/BB dropped from 3.5 to 2.5. His velocity has fallen each year since 2010.
Ok well we just look at pitchers completely differentlyt hen. His K rate drop was almost not noticeable in 2011, I'm guessing you are looking at K/9 which isn't very useful, his K/9 got much worse because he put fewer runners on. HR/FB% is the least reliable of all pitcher stats, his GB% improved so he got better. 2011 he was a better pitcher in every way than he was 2008-2010. His velocity was at its highest in 2011 so part of this isn't even true and in 2012 it was down in April and right back to normal the rest of the season. His average his last game was over 91 as well. 2012 was a mirror image of 2011 outside of 2 bad starts in early April both against the Cardinals, that is the only real difference between the two years.
The 2013 samples are worthless so far so can't speak to trying to make much out of those. In 2011 his K/9 was all of 5.7 in April too and that was his best season, in 2008 it was 5.7 in April and 6.0 in May. He just doesn't like pitching in the cold.