It depends on how heavy your right foot is on the accelerator. On the flat, the car does a smooth acceleration. However, on an incline, a human would ease the car up to speed slowly, knowing it was an incline, whereas the car would try to accelerate at the same rate as though it were on the flat, so being less efficient. Also on a down hill, a human would probably let the car roll along to its cruising speed, where the car would still apply power to accelerate.
Swings and roundabouts.