I think the persistence of the myth is at least partly due to the fact that it aligns well with informal observations.  You have a class, and some of the students have difficulty absorbing the material.  So those students are taken aside and taught to in different ways, which are, sometimes, successful.  So, informally, people come to believe that those students were better reached by the alternative methods, when in reality it may have been the repetition, extra attention, or something else that improved their scores.