I can understand why it is impossible to give a exact time. Besides file sizes ect (which can be seen in advance) you also have a load of other factors the change during copying. If you copy and during the copy you start a program, there goes your estimate. Strictly speaking a change in outside temperature can influence the computers speed and thus the copy-speed.

But somewhere someone decided that, although it would be an estimate, some calculation will be made.
And what that calculation is, that is what I'd love to know.

I think it is based on the copy-speed of the last completed file, based on the behavior of the estimate. But I am only halfway sure.