Unless I'm mistaking, what Ozplane is asking, is why does the engine not return to 2450 rpm, once he removes the carb heat? ie. why has it lost 40 rpm, by only returning to 2410.
I don't know the answer. The only guess I can make, it that the aircraft has slowed a little while the carb heat was applied, because of the reduced power. Then, if it's a fixed pitch prop, it won't quite develop the same rpm until the speed of the aircraft through the air has caught back up to where it was.
Having said that, I'd be skeptical if that would make the difference of 40rpm.
dp