Does the authors comment "are so many doctor Dans simply because Dan is a common name", not opposed to the methodology used "men with unusual names, the study found, were more likely to have flunked out or to have exhibited symptoms of psychological neurosis than those with more common names."

In the first study, they weren't looking at the number of "Doctor Dans", they were looking at a subset of people and placing them all along a graph and watching for trends in performance not based on a name, but based on the popularity of the name. These strike me as different things, but I'm no expert in these (or any) type of study.