So the way the adcom member explained it to me last year was that its based on the type of applicant. For example: Imagine two applicants,
Applicant A has a 504 MCAT, 3.7 GPA, lots of clinical volunteering, specifically with leadership, but no research.
Applicant B has a 508 MCAT, 3.9 GPA, not a lot of volunteering or leadership, but lots of research and publications.
If a person who has a similar application to that of Applicant 1 decided to decline their acceptance, then the next person they would choose on the waitlist would be Applicant A. If it was a person with a high MCAT/GPA/Research that decides to decline then Applicant B. This ensures that the "diversity" of the class would stay pretty fixed.
Hope that helps!