There are some specialties and some specific residencies that so not use the NRMP program.
For almost all residencies in opthamalogy, neurology, neurosurgery, ENT, and combined plastics there is something called the San Francisco Match which happens a few months earlier than the NRMP. So the programs you see on the NRMP site are the few that participate in the NRMP. Go to <a href="http://www.sfmatch.org" target="_blank">www.sfmatch.org</a> for more info.
Some of the stats you mention, like rad onc and derm are for programs that include the first year of training in general medicine or surgery(preliminary spots or transitional years). These specialties, along with radiology, anesthesia, some ER, PM&R require one general year followed by training in that specialty. If you go down a little further on the statistics page you will see programs in these field in which people matched for second year spots. These people will do their first year somewhere else. So, to get the total number of spots add up the combined and non-combined programs (e.g. rad onc 14 + 83 = 97 spots).
Hope that clears things up.