A lot of civilian colleges will usually accept well over their target/limit for freshmen class size because they know not everyone will ultimately matriculate to there. They have the yield stats from year-to-year and account for that in their acceptance decisions.
@MidCakePa hit the nail on the head. It’s not always a 1-for-1 exchange of offer declined = someone else gets in. Now if it starts dipping below their target class size because of too many declines, then they’ll be going to the waitlist more and more. They’ll also go to the waitlist when trying to round out those last couple slots around May 1
Sometimes with the Ivies, if you get into 1, you get into some of the other ones too. The schools are well aware of this. At my school during admitted students weekend, I remember a lot of people were choosing between offers from us and the other Ivies (and Stanford/MIT/Cal/etc). They also collect information on which school you chose over them if you ultimately decide not to matriculate to them. From what I’ve been seeing on this website, it could be the same for the SAs where a candidate often gets into multiple academies in a cycle