That's a function of trusting LCME accredditing over COCA accreditation. Not everything comes down to grades and board scores. The environment in which those scores were obtained is also important. Furthermore, PDs are more likely to take students from schools that they have experience with, especially if past students turned out to be great residents and since there are more MD schools and PDs are more likely to have a longer history with MD schools, they'll trust the student from the MD school more than the DO.
However, despite all that, many DOs do get into ACGME residencies and do well. You just have to aware the bias exists and deal with it. Sure, everyone makes mistakes, which is why DO schools have grade replacement and such, but at the same time, mistakes have consequences.