Waitlist movement is not some hard and fast set of criteria. At many schools, if not most, WL are at least generally categorized into high, medium, low. There is also at many schools, a less formal process in coming off WL. To explain, everyone on the WL has passed the adcom review as qualified/accepted to medical school. They simply didnt enough open seats to offer. As seats do open, either the committee as a whole, perhaps only on the first large wave of WL, will meet to approve, but usually it is a subcommittee, adcom staff, and/or key members. This is also often where the 1 or 2 "pet" acceptees of members (ie someone they really liked) who didnt get in first round might get pushed by their advocates. As the WL cycle goes on, it becomes more chaotic as you may a spot open up but the first 5 people on WL have accepted elsewhere, have to wait until they reply back to accept or not. Some schools, though not that common, may look at where the next WL has been accepted to see if they have accepted to another "higher ranked" school. By the middle of the WL cycle (mid-late June), any ranking of WL people is just thrown out of whack from this chaos, hence why most schools just use broad categories. As the late cycle goes in July and August, often schools take whomever they can contact first on WL. In the "old days" (before email and cell phones) I know of a couple of doctors who got phone calls just 2 or 3 days before classes started. Why where they chosen? First one who picked up the phone or they lived close to the school.
Sometimes it is just luck of the draw
