In addition to the day warming up after you set the pressure there's also the fact that tires get warm when ridden. 5PSI worth? Probably not on a bicycle. More than that on cars or motorcycles.

Probably mostly the day's temperature warming up from the start to end of the ride. Got up to 85 by the end of the ride? What was it when you pumped and checked? If it was down around 70 and warmed to 85? That could be the 5PSI.

The other option is if you used the same gauge for both measurements. Different gauges often give different readings. I'm thinking same gauge though since we're creatures of habit.