A very basic back-of-the-envelope math in Excel shows that accounting solely for temperature changes (so assuming constant humidity), if the temperature on the field was 50F, a ball inflated to 13.5 will still be at regulation pressure if it was inflated at temperatures of 68.5F or cooler.
Correspondingly, a ball that was inflated to 12.5 will lose 1 PSI if it were inflated at 69.25 degrees, and 2 PSI if it were inflated at 90F.
My point is that I can easily see the Pats taking the ball inflation process to, say, an 80F laundry room, which is likely to result in about a 1.5 PSI drop in game conditions -- something that is technically not against the letter of the law, but to which the team will never admit publicly.