Just guessing -- because he has big hands so it doesn't effect his grip and over inflating makes the ball harder and, thus, more difficult to intercept.
If it was more difficult to intercept it would also be more difficult to recept, and his stats don’t seem to show his receivers have a problem. Again, all the evidence indicates it’s nothing more than personal preference with no actual difference. Some people like loud keyboard, some like quiet, some like clutches that engage high, some like low, some QBs like overinflated balls, some under. Doesn’t matter what Chris Carter, a receiver who played his entire career with factory default balls provided by the league, says. The facts speak for themselves. And the facts say if there was a verifiable performance difference in one direction everybody would be doing it, they aren’t, so there isn’t.