I should not have said that I didn't want to discuss the issues. The truth is, I don't really understand them myself. I think the reasons for graduation time vary by individual, but the reasons that the averages vary is because of the philosophies at different schools. Without going into much detail I think it has to do with three factors: strength of advising, requirements (TAing, rotations, courses, etc), and integration of MD and PhD cirriculum to get around requirements on both the graduate and medical school sides.