This is exactly how it works. I go to church with a gentleman that used to work for Disney in the department that would analyse and set up all the promotions. This is how I found out about the FD the first time they offered it in 2005! Sooo, when I saw this post, I decided to give him a call to see how it worked. We just hung up and he said the way it was done when he worked there (before he retired) in 2012: When they start finalizing the promo's, they DO an analysis of available rooms at that time for the date range and every resort they are considering FD for. They have the "original" amount of rooms they need to fill for each day/week and have the preliminary # and they know which resorts will/will not be included at that time. They do analysis periodically all the way up to the DAY before FD is released and that is when the FINAL amount of rooms are set to be available with FINAL numbers. He said, within that time frame, it is crazy how many changes they make; room availability and even adding or removing resorts entirely up until the very last day. He said the room availability, once it is time for the FD drop, is set; then the analysis is done all over again on a bi-monthly basis after FD rolls out, until the last day that its offered. With rooms being canceled, moved, etc there are times they open a few more rooms on certain dates and resorts and other times there is availability that opens just because of the cancels and moves. Bummer he doesn't work there anymore or I would have a lot more information for all of us!
But you are correct believing pre-booking causes less rooms to get the promo! It helped me to decide, not to book until FD drops even though we are going with or without FD. We may not get the resort we want (2 rooms at POP) but I will check and re-check to see if anything opens up and ultimately we will be at Disney in December!
Good luck to everyone!
Pixie dust that we all get what we want (or at least close)!
Probably the same reason as to why UK gets to book FD a year ahead of us 
