I don't think there's any quota anywhere. Adcoms look at the applicant pool and take the best they can get where there's a good fit. I'd be surprised if postbacs actually made up 40% of any med school class, and it's doubtful that that is an annual statistic if true. However it's possible some postbac program directors likely have better relations with certain deans than others, and are able to market their students more effectively at some places. Other deans may highly regard certain postbac programs based on prior matriculants' quality or success. It should also be noted that since NYMC is at the lower end of the spectrum in terms of competitiveness, it may just be a popular school for folks who rehabilitated their applications with a postbac but not to the point where they were competitive with higher ranked schools.