For example, lets say Chicago has 3 flights to Frankfurt(i know there are more, i am just improvising).
The first flight leaves at 10 am, the second leaves at 3 pm, and the third leaves at 7 pm. The flying time for the 10 am is 9 hrs 35 min, the flying time for the 3 pm departure is 10 hrs 30 min, and the 7 pm flight is 9 hrs, 20 min.
The reason the 3pm flight is longer than the 7 pm or 10 am flight is that there is more ground traffic and longer takeoff lines.
The flights will probably be in the air for around the same amount of time, but when there is more groud traffic, that means a longer flight.
The flight time starts the second the gate is pulled away from the aircraft, not when the aircraft leaves the runway or reaches its altitude.
Give me a break, I created this username when I was a kid...