My guess is that, since the study the Center for the Study of College Fraternities cited was done by the North American Inter-Fraternity Conference, they only looked at GLO's that were/are members of that group. That, of course, would not have included a member of
AFW. It would also have also excluded a brother in Phi Mu Alpha... perhaps not much longer, though.
Using a broader net than just the IFC might mean more Greek presidents, but I don't know how many more.