Great question — answering as the UphillTech team:
Flight schedules are often padded somewhat based on historical performance. Airlines know if a flight is delayed frequently and can adjust their schedule accordingly. As such, it's not uncommon for a flight to leave somewhat late and still arrive on time, or at least not as late as the departure delay alone would indicate. If the flight schedule represented only the best-case conditions, many flights would be delayed frequently. This, of course, means the flight may arrive early on days when all goes well, which passengers rarely complain about, but airlines hate, as it means aircraft are sitting idle, reducing profitability. Airlines can adjust speed to a small extent , which helps make up...
Full answer by Zach Lipton: https://travel.stackexchange.com/a/86547 (CC BY-SA 3.0).
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