That's interesting. I hadn't heard about a pool. To go along with that, I was always told that each congressional critter has 5 nominations, total. So 4 are usually taken up by those already "in the system" and the 5th is for the applicant. If someone drops out, then they gain another nomination.The congressional nomination is probably the most misunderstood part of the academy admissions process. All it really boils down to is your congressperson submitting your name to Admissions for consideration. They can nominate a primary and alternates, or just send them a pool of names That's it. Admissions then looks at your record and decides whether to offer you an appointment. It's a holdover from the days when they were trying to avoid West Point appointments becoming a political patronage tool, and get officers from all over the US instead of just whoever happened to be friends with the Secretary of War (this was a real problem in the early days of the Republic - one of Meriwether Lewis' first jobs for Jefferson as his military aide was evaluating the political reliability of all the officers in the Army). USNA adopted the same system when it was founded.
Weird things can happen in the process. I didn't get a nomination from my Congressman (the easier one to get) but then later was given one by one of the two Senators. I don't really remember the details, but I'm wondering if some deal was made across all three offices to move around competitive applicants and maximize the numbers. Ironically, the applicant that one the congressional nomination didn't attend and I later ran into her at ROTC...which she eventually quit. So probably good headwork on her part not to accept.