Also just to let you know, there are three different billing departments. One for the physicians themselves and their portions, one for the hospital facility use itself, and if anesthesia is required for your procedure, there will be a bill from the anesthesiologist too. Each has their own sets of billing procedures to some extent. I really only know of the physician side.

If i were to speculate for the hospital side, there is a chance that if your surgery isnt considered serious enough, they could be requiring that payment for the timeslot up front, and if not paid they would move a more urgebt surgery into that time frame. This is purely speculation as I have little to no idea on that side.