I'm currently an MS4 at of the "Big 3" Caribbean Medical Schools so maybe I can maybe also shed some light on this discussion.
Going to any of the Big 3 vs any of the other Caribbean medical schools is already a bit of a disparity. The big 3 have been established and have had a track record for quite some time so they're a little more organized and hospitals and programs know about them, thus making it easier to match.
These schools will always be sustainable because USMD or DO grads don't apply to the same specialties or programs within specialties. The reality is, most grads don't want to be an FM doctor. "Commitment to the specialty" is one of the highest-rated items from the NRMP PD survey, and after discussing with my USMD and DO friends, the only ones who were applying for FM were pretty much only doing it as a backup. PDs want residents that are passionate about their specialty. Best example I can give is how I have proportionally more interviews than my DO friend, mainly because his personal statement and extracurriculars are more about his #1 specialty(which his scores are not competitive for so he, unfortunately, has not gotten many interviews for) so its quite obvious he does not want to be an FM doctor on paper. I've been set on FM since basic sciences, so on paper, you can feel my passion for FM.
Even within specialties, if you look at the match lists, by and large, we match at the same hospitals and similar ones each year. Our match-list for Rads and Psych were better these past few years, but personally, I believe it's because more Rads and Psych residencies have opened up at similar programs we match at. Most people don't want to be in the deep rural, or at malignant programs in desirable areas, but most of us are willing to go there.
I think a previous poster also hit the nail on the head. Some of my class, to be honest, are just not good students. A majority definitely are, but a certain portion, just don't study enough. 1st semester, there were students I knew who would wait until the week of exams to start studying. Comparing my experiences with my USMD and DO counterparts for basic sciences(because after that for clinical we're back in the US, where I personally noticed no difference) I noticed that it was a little bit harder than they had, but there was not as huge a disparity as I thought there was going to be.
Anyway, that's just my $.02 in the situation