Ah ok, that does make sense. I do remember on more than a few visits seeing one of the girls fully clothed laying on the sofa looking bored out of her mind whilst the other two were walking around nude/in underwear getting ready for the next client.
Although at first it seems strange to have 3 girls working but only 2 rooms, it could make sense if the girls are different, for example different hair colour, size/shape and ethnicity, etc. as this would perhaps increase the potential for bookings. There's also the question of availabity of 3-bedroom flats and the cost since 2-bedroom flats are easier to find and cheaper.
From my experience with HOD, they will allocate the girl a room at the time you make the booking so, even if your chosen girl may not have a booking at your preferred time, you will be out of luck if both rooms have already been allocated.
I can also see why some girls do not like to work at the same time as one or two of the "superstars" since this must have a big negative impact on her chances of getting a booking.