In NRMP's charting outcomes data, there is a table where they correlate the number of programs applicants rank and the probability of matching. I noticed that even for the most competitive specialties, the probability of matching is almost 100% if one ranks more than 18 programs. Why are there still unmatched graduates when one simply has to rank a large number of programs to ensure matching? I guess there's no meaning in ranking a program if one isn't invited for an interview, and the number of interviews received could be the limiting factor. Is there anything that I am missing? Thanks a lot.