[somebear]

FTA:

> Some of the available energy is lost as heat and in the internal rearrangement of chemical compounds and other changes in entropy.

So not all of the difference in efficiency is made up in the form of heat. Unfortunately, I don't remember enough biochemistry nor thermodynamics to dive deeper into this, but I think this article is a good starting point for a discussions, which has otherwise just been dismissed with the "a calorie is a calorie" argument.