On what basis are they decided?
OP has shown a very scientific approach to other issues, so I'm sure he has used some kind of points system, but I am unfortunately not aware of the details. One can read certain things from the list, though. It is obvious that his focus is singles - doubles do not count. Also, he has no era prejudice, majors won in the 19th century counts just as well as later ones, but he does not care for the professional results before the Open Era started in 1968. He is not interested in major results only but include results from minor tournaments also.