Some girls are super popular and book up ahead of the time they are scheduled to start. Some could have also had a change of plans and instead of working a 8 hour shift she told the booker she was only working 4 instead, lots of factors here besides just the old "the agency is playing with us" not to say that does not also apply at times. There was a agency years ago that was famous for that and I am sure some still do and right or wrong it is business sense 101. Also some bookers take the girl off the schedule once she is fully booked, it is common with some agencies and not with others.
Even then, the agencies will play with us... They will advertised thoses girls and pics in threads and if you call and ask for that girl they will answer you "full booked" hopping you will ask for another one.
It hapened again to me last night... I called the agency for one girl one hour before her shift start to make shure I get her during the evening... The girl didn't even started her shift and she was alleready full booked. Strangly a short time later she was not advertised in the thread any more.
Steve