While what you're saying is in line with what we see a lot in the real world (e.g., in business, you can only make one deal when you have two on the table), it's strange to believe that this would also happen to med school admissions. That is, a candidate's value is only realized when another higher-ranked school also recognizes it, i.e., it is not enough for a school to trust its own judgment about a candidate's qualifications without confirmation from some other highly ranked school.
Somehow it comes down to this for some med school adcom:
1) Waitlisted candidate who didn't apply to any other higher ranked-school
2) Same waitlisted candidate + acceptance offer from higher-ranked school
#2 is more desirable than #1 due to circumstance and not because s/he is inherently a great candidate, even though they are the same in each case!