I will say I've been on a number of flights that left late but arrived early/on-time. I think they build a pad into flight time and/or give up fuel efficiency to make up time.
There was an article posted to HN recently (today, even!) that showed how airlines do pad their flight times deliberately.
I think it's reasonable. Even if you depart the gate on time, there could be things out of the airliners' control that cause delays, such as a long queue to take off.
