This is true, which leads me to think that how 'strong' an opponent is (hp, ep, etc) is not a very good measure of how much experience you get from fighting them.
It would seeme that a system measuring the length of the fight, how much damage each participant took, if the fight was balanced, and how close you came to losing would e a much better way to calculate experience gained.
So, you start a level, battle your way through it for hours, nearly die several times, and of course you get a ton of experience. Or, you throw fire at everything, run from start to finish in five minutes, and get almost nothing.