I think the difference basically comes down to this, and I'll be completely candid.
West Indies was set-up to address a need in the area, and to train physicians to practice medicine in the Caribbean. Ross and SGU (and others) were set-up to address a need in the U.S., and to train physicians to practice in the U.S.
These are two completely separate goals being pursued here. At Univ of West Indies, they will strongly consider students who intend to become physicians and remain in the islands. As a result, they predominately draw from those students who are native islanders, and the school is competitive. At Ross and SGU (and the others), they'll strongly consider anyone, and they predominately draw from those who - for whatever reason - couldn't go to school in the U.S.
You can't fairly compare Ross and SGU to UWI anymore than you can compare Ross and SGU to a school in Western Europe. Their primary missions and reasons for existence are completely different. And, if you are not an islander with a competitive application and/or you interview and tell them flatly that you intend to come to the U.S. to practice when you graduate, you will likely not get a spot there unless they have extra space that year.
That's not to say that a strong applicant from the U.S. wouldn't do well there any more or less than any other student, like they would at Ross or SGU (or the others). But, they don't offer a lot of admissions with the intent of subsequently weeding people out during the education process. Again, their mission is to provide doctors who want to practice in the Caribbean. It's a hard program to get into just like U.S. schools, and they have a completely different educational modus operandi.
Put simply, this school does not exist to cater to U.S. students like Ross and SGU (and the others) do.
-Skip