As someone who used to be a rooms controller for the ASR's, I can say at least during my time working there, it was definitely harder to meet someone's requested room types as the day went on. Not saying it's not impossible, but the available rooms become limited as more and more people check in.
Think of it as a game of Tetris, you have x number of reservations checking in every day for different lengths of stay, and you have to try to find the room the guest wants that fits their requests. It can be tough, and sometimes the front desk would offer to do room changes if we couldn't meet their requests they wanted and were okay with moving the following day.