To reiterate what others said, the fact that 86% of PGY-2 positions were filled by US seniors means nothing if the remaining 14% were filled by those who just completed a derm research fellowship or (like several people I met while interviewing) were currently residents in very competitive programs of another specialty, but who decided they wanted to switch into derm late and were really awesome applicants.
While a high rate of non-US seniors is often translated as an indicator of being non-competitive (because the spots are instead filled with IMGs and DOs), it is meaningless (or may actually signal, in effect, the exact opposite) if high non-US senior fill rate is simply because it takes some US students additional years of research or clinical work to get into a field they find highly desirable.
A statistic of possibly more useful consideration: for the 369 PGY-2 spots offered, 442 US seniors applied and 316 of those US seniors matched. 316/442 = 71%. From this, you could roughly estimate ~29% of US seniors applying for derm did not match. This does not take into account the overlap that may exist with those students who also applied and were accepted for PGY-1 derm spots, but even if you factor this in and use the most conservative estimate, it only shifts the unmatched US senior rate down by a few percentage points. And as others have pointed out, these people who don't match are of a significantly different caliber than the typical unmatched applicant found in some other fields. Lastly, of students who apply to multiple specialties (derm + IM) and therefore are counted twice in the # of applicants field but can obviously only match into one field, nearly all preferentially ranked most derm programs over most programs in the other specialty. In other words, of those who ranked derm plus [any other field], most likely didn't NOT match into derm because it was there second choice specialty or because they were using it as a backup option.
