I don't think it's the temperature delta in this case, at least not between the room and outside. I suspect internal equilibration of the unit when power cycled and also energy transfer design capacity on the indoor side of the unit are to blame.

> Most of the consumer advice is very low level - this Heat Geeks site seems better that most, but it still lacks enough detail for me to get into the numbers

You'll probably want an undergraduate physical chemistry textbook for this.