Let's pretend a school has 100 seats for a class:
Interview 1: 20 interview: Give away 20 seats
Interview 2: 20 interview: Give away 19 seats
Interview 3: 20 interview: Give away 18 seats
Interview 4: 20 interview: Give away 17 seats
Interview 5: 20 interview: Give away 16 seats
Then all the post-December interviewees will fight for the remaining 10 spots and need to have apps that are stellar. These are your 25AA, 100,000 volunteering hours, curing dental caries, etc.
The amount of invites don't change, the amount of seats left change. Schools are no longer generous because everything has been given away. This is an example an admissions person shared with me. She even told me that applying close to the deadline is wasted money because they already planned on all the interview invites they want to give post-December.
Good luck and congratulations on your first interview! Keep a positive attitude and vibe and you'll get in!