I have no scientific evidence, but I guess what I'm trying to say is that on a steep hill, your speed drops off faster due to gravity, compared to on the flats, so when you are climbing, torque is applied earlier during the pedal stroke, and maintained for a longer period of time to keep the speed up. On the flats, the wheels don't slow down as quickly, so you can apply all the torque over a smaller period of time to top off the speed you are trying to maintain.

Does the application of torque over the pedal stroke remain identical no matter what the terrain is?
Again I'm just thinking out loud, could be completely wrong. Feel free to school me.