PM&R is a small field that isn't very mainstream. They pretty much just handpicked a few of the more well known specialties. Other fields that weren't mentioned were rad onc, ENT, neurosurg, pain medicine, path, CT surg, vascular surg, etc.
If you look into the methods... the n is extremely small. I think its something like 26. Most of the specilaties represented were based ona single input for each maybe 2.