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It could just be a case that most of the sub-50% people are significantly overlapped with the students that also didn't get a competitive Step 1 score for Ortho in the past. I don't think that's too far stretched.. In this case, the OP said the last time someone matched from the bottom 50% was 2011, so it seems like it is very difficult to overcome from his institution.
Also, @everyone , couldn't this debate just be solved by looking at the Charting Outcomes PD survey and check how many programs care about AOA/class rank?