I dont mean to make it sound like DL adds crazy padding. However they lean towards scheduling so all flights come out on time/early. They schedule so that all flight factors that could be faced are accounted for all the time. If even one day during that schedule a flight has a chance of being 5 mins longer, they schedule all days 5 mins longer.
It's a common read on here to see DL flights arrive early, and for delayed departures to arrive on time.