Departure time and arrival time as quoted by airlines is departure from and arrival to the gate, not just flying time once airborne. The amount of time allocated for taxi, waiting to take off, waiting for an arrival gate, etc can be substantial especially when you are talking about congested airports which most of the major international hubs are. Also long haul flight times can be affected by time of year and typical winds. For all of these reasons the posted flight schedule includes a discretionary amount of "padding" on top of the actual flight duration. It's most obvious on very short flights (eg Seattle to Vancouver) when the posted duration might be twice the length of the actual flight!