If you have a longer crank are you have more leverage and can push a larger gear. If you are more inclined to hammer larger gears (i.e. accelerating from stops, climbing out of the saddle) the longer arms give you more leverage. If you sit and spin the longer arms tend to create a more choppy spin.
Watch the stop watch on your computer. That will usually give you an insight into what triggers the average speed calculator as the stop watch should stop also.