You're drawing conclusions based on this alone? WTF? I doubt, for example, Cornell Weill, a pretty much all-PBL school has low board scores. I'm not that much of a fan of PBL in the first place, but your conclusion based on this point is what's BS. First off, correlation is not causation, and second, the correlation is pretty weak in this case. As for the differences between the campuses, perhaps there are different admission standards for each campus correlating with the board scores. Mandatory attendance, maybe, I can see... if you don't go to class, you're forced to develop your own discipline, so that may be a factor -- of course the percentage of people in the class who actually develop that discipline might vary, etc.
There are so many other factors that have a much higher probability of affecting the board scores, such as administration of shelf exams, "teaching to the boards", a higher amount of time off to study before the boards, school-given practice exams, etc., any/none of which might be relevant here. Of course you are not a medical student so I suppose it's somewhat understandable that you not consider anything else. First you call "shenanigans" and now this?
Also, there are many other schools which do not report their scores... Penn and Baylor are reported to have averages in the 230's as well. Reportedly, integration of subject material between traditional disciplines was cited as one of the factors that resulted in higher board scores. There are plenty of documents available from the AAMC which should shed more light for you on medical school curriculum / the USMLE.