From interviews this past year I found that there are programs which have multiple spots and they fill some by the match and some not through match. I know for sure that both NYU and Kings County have a 4 year non-match position (they each have 2 six year match spots). These non-match spots are not advertised anywhere, and I think they are used for the program to take in their interns (if they want to) - especially the 6 year programs where the intern has already done 1 year and would rather enter a 4 year program than do 6 more years.
Im sure there are more like that than the two programs I mentioned, but as far as I know, evey program that offeres a non-match spot does participate in the match for some of their positions, and the non-match ones are kept on the DL.