I think and someone can correct me if im wrong but when oil is warm it is thinner so there 4 it will sit more compact.
When it is cool it is thicker there 4 it will sit less compact.
I know it doesnt make much sense but i hope you get my drift.
I also think there needs to be a certain amount of oil in the sump at all times, so there needs to be a certain amount in the engine so that you dont under-fill the oil.