The AOA Match is through a different agency (in this case, the National Matching Service) than the ACGME match (which is through NRMP). Hence you're only seeing the AOA Match statistics.
For 2013, PCOM was expecting 271 graduates (who are eligible for both matches - NMS or NRMP). In addition, 30 additional applicants who graduated previously from PCOM were also entering the AOA/NMS match - for a total of 301 potential applicants from PCOM for last year's AOA/NMS match. Of those, 123 did not participate in the AOA/NMS - most likely they are the ones putting all their eggs into the ACGME/NRMP match.
If you look at other schools who traditionally have more of its graduates go into ACGME residencies than AOA residencies, you see the same trend. If you look at TCOM/UNTHSC - they have 184 potential applicants in 2013 (179 graduating, plus 5 from prior years). Of those 179, only 33 participated in the AOA match, with 151 not-participating in the AOA match.
I wouldn't put too much stock or read too much into how many people who did participate in the AOA/NMS match didn't match. With the nature of having 2 matches (NMS and NRMP) at 2 different times (February and April) - each individual applicants have to decide if they want to put all their eggs in one basket (whether all AOA/NMS or all ACGME/NRMP), or if they want to put eggs in both. And each individual will have to decide how to split up their rank list. So a high percentage of unmatched NMS is not necessarily an alarm bell. What would be more meaningful is the number of students who remains unmatched after NMS and NRMP (and have to scramble for a spot). Unfortunately I'm not aware of any aggregate data from schools on number of students who need to scramble.