You are making a few assumptions of your own. Who said they ended at 13.5 psi? It is more likely that the Patriots balls were deflated while the Colts balls were not.
You’re half right. I mangled my statement. Thanks for bringing that to my attention.
Let’s try again: if the balls started at 13.5 PSI at 75°F, they would still deflate below 12.5 PSI upon the air inside them cooling to 50°F, as even without water being added, just with the effects of the air cooling alone, the drop would be 1.2 PSI which is more than the entire range of allowed values. Thus the Colts balls could not have both cooled 25°F, and both started and ended within specs. It’s even moreso when the effects of the water on leather are taken into account for the balls which were used.