@RangerBob nailed it.
In regards to the DO match information above. You do realize that those COMLEX numbers are only for the DO Match and not ACGME match. Do you think that there are were only 18 DO's interested in PM&R last year? No way. Practically every DO interested in the specialty deferred the DO match because it would have screwed up the opportunity to match ACGME. I'm a DO...I didn't apply DO. I only personally know one person who applied DO last year and it was after the person failed to secure more than a few interviews with ACGME. DO PM&R is NOT competitive...ACGME is.
I think that your definition of competitive is flawed. A competitive applicant isn't always the applicant with the best board scores. If I was an internal medicine program director...absolutely...USMLE would be very highly weighted because it is primarily a test of Internal medicine knowledge and application. Do you need to be brilliant at Internal Medicine to be a great PM&R doc...not so much, though of course it doesn't hurt. PM&R residency classes are filled with applicants, like me, who found ways to be competitive applicants in more ways than just being able to take a test. I very likely took the seat of a couple hundred applicants with better board scores. It's not that people with high board scores aren't applying, it just that PM&R doesn't always take the guy/gal with the best scores.
If you prepare to match to PM&R with the thought that ACGME PM&R isn't competitive...