Take a look at this photo and story on the inflation of balls. That does not look like a precision inflation nor calibrated system. And unless the exact same gauge was used, this whole discussion is subject to significant error.
Deflategate: Video showing how officials check pressure of game balls | The MMQB with Peter King
Also, I read somewhere today that most of the balls were 11.5, one was 10.5. 11.5 psig is possible from a temp drop from 70F to 50F, since they were inflated indoors and played with outside.
For the poster talking about the change in air pressure, the barometric pressure dropped from 29.96 inH2O @ 12:53PM to 29.75 inH2O @ 6:53PM, while the outside temp stayed the same at 52F.
If the inside temp was 70F and a ball was inflated to 12.5 psig, that would be 530R and 27.2 psia.
52F = 512R. So 27.2*512/530=26.27 psia=11.57 psig
Now, depending on the prior inflation of the footballs balls initially, the compressing of air COULD have raised the internal temp of the ball beyond the 70F.
70F again is 530R. Inflating to 12.5 psig from atmospheric, the temp of air going into the ball could be as high as 530R*27.2/14.7=980.7R=520F. I doubt it was THAT hot, since the compressor has a heat sink. But no doubt the air inside the ball was above 70F.
So YES, it is entirely possible they could have inflated balls to 12.5 psig and they dropped to 10.5 during the game.