There are more girls than rooms in most places, you can’t expect the girls to be fully booked the whole day. 2 reasons for this, 1 is to avoid down time of the room, 2 is to keep more good girls on the roster. If one girl occupies one room for the whole shift, she has to be popular enough to make the full use of the room, even then they have to shorten her shift to allow another girl to work in that room.