I think they all do it differently. Some may meet the day you interview to discuss, and they probably have some sort of scoring system.
I know Columbia waits a week after you interview to hold the formal "discussion," so they don't do them all at once. I'm not sure that they make the decision right then and there because they still have other people to interview, but I imagine there's some sort of scoring system that would place you in a certain category?
I don't know how every school is, but I know some don't wait until a couple weeks in December, but instead hold deliberations throughout the interview process.