Looking at the schedules on-line for Air Canada from YVR to YYZ, I noticed that the travel time listed for each of the flights is different (e.g. looking at all the flights on one particular day) - even for flights operated using the same equipment. The difference is really small (+/- 5 minutes: AC3993 vs AC3997), but can someone tell me how they come up with the flight length #s reported on the schedule? Are they averages from the previous scheduling period? or built in estimates of ATC delays based on time of day?