Yep, also some defer for scholarships and service projects (Fullbright, Rhodes, Peace Corps, TFA, etc.). Here's some fun data from US News:
School: Accepted/Enrolled - Reciprocal Percentage
Harvard: 208/165 - 79%
UPenn: 222/154 - 69%
Columbia: 291/153 - 52%
Duke: 218/100 - 46%
UWash: 267/216 - 81%
WashU: 346/122 - 35%
JHU: 268/118 - 44%
UCSF: 257/152 - 59%
Yale: 245/100 - 41%
Stanford: 172/86 - 50%
I think what's really interesting is the LACK of multiple acceptances. I mean think about it: Almost every school listed above [except WashU] has a >40% matriculant rate for accepted students. I'd think that if it were the same 230 or so students being accepted everywhere that each school would have to accept many more than they'd matriculate. The data above indicates that schools can deduce whether a student will actually attend their school within a 40% margin of error.