BTW the physics is correct, you could theoretically get all the energy back you put into climbing the hill. Unfortunately, it just doesnt work that way. The human body is not a perfect adiabatic engine. You end up wasting all kinds of energy generating heat, friction in your muscles, chemical inefficiencies, etc. You are not operating at your highest efficiency when you are climbing a hill and your body is not requiring 0 energy while you are going down. Throw in wind, braking, rolling friction, etc and your average is definitely taking a hit when you ride through hills.

Of course, we all know that looking at averages is pointless anyway...