The “padding” is based on actual flight times of previous flights between city pairs. It’s a sample size of I believe 1 year or the historical flight times for that given time of year. I just don’t remember exactly. Regardless, the point is that all flight times published are completely data driven.

Crew pay, duty limitations, fuel planning, the augmentation of flights are all predicated on published block times. United doesn’t just willingly add a few minutes just because.