I dislike the lack of sample size here. I’m not saying you’re obfuscating the truth, just that I’d like to know that Gio’s thrown 877 pitches. 7% change is what, 60-ish pitches? One offspeed pitch per inning in 2011 turned into a fastball in 2012.
Pitch values say that his fastball has gotten a LOT better. They also say that his changeup has gone from being not very good to being quite good. I can imagine you could make an argument that a better fastball = more fastballs = better change, but why would his fastball be better to start with?
One thing that sticks out about Gio’s K rate this season is how much it is aided by facing opposing pitchers. Following the Atlanta game he has recorded k’s on 15 of 19 PA against opposing pitchers (plus 2 sac and 2 GO). Maybe there’s something about his stuff that dominates non-mlb level hitters.