People like to say the match rate for US-IMGs is 53%, because that's what is listed in the NRMP reports. This number does not refer to first-time match rate (i.e. recent graduates applying to residency for the first time), which is what people are commonly referring to when saying "match rate." This is exemplified by the fact that the NRMP data reports actually breaks up US allopathic applicants into US seniors vs. US grad.
If you look at the ECFMG report from 2013, you will see that the average time since graduation for the unmatched cohort of US-IMGs is 5.7 years.
When quoting that 53% you are not actually describing US-IMGs in the match for the first time, you are describing all US-IMGs in the match regardless of how many times they have applied. The NRMP data reports don't differentiate US-IMG senior vs US-IMG grad. There is without a doubt a percentage (~10%) of US-IMGs that manage to graduate from school, but are poor applicants (semester failures, step failures, etc) and are not able to ever match. This small yearly cohort continues to apply every year, builds up over time (hence the 5.7 years above), and drastically skews the US-IMG "match rate".
If you want to include those reapplicants when describing true US-IMG "match rates", then you also have to include all the applicants from those previous years that did successfully match. If you don't do this (like the NRMP data reports), you are way oversampling the poor applicant/unmatched cohort. The NRMP does actually recognize this because they split US allopathic applicants into those 2 groups, they just don't do it for IMGs for some reason.
People try to say the US MD vs US-IMG match rates are 94% vs 53%, which is in fact comparing 2 completely different data sets. The yearly match rates for NRMP defined US grads (i.e. non-matched applicants from previous years) are actually 40-50%, showing that poor applicants, regardless of where they come from, do not do very well in the match.
The first-time match rate for all US-IMGs is ~75%, and from the big 3 caribbean medical schools is between 80-90%. Granted this does not take into account students lost to attrition before graduation, which is no doubt substantial.
But when people say caribbean grads have a ~50% chance of matching, they are grossly misinterpreting the data.