The list was created by the same people who collected and organized this database, in consultation with university professors of comparative religions and scholars from different religions.
A listing of doctrinally and organizationally meaningful divisions or denominational "branches" (such as Catholic, Eastern/Orthodox Christian, Sunni Islam, Shiite Islam, Evangelical Christian, Mahayana Buddhism, Theravada Buddhism, etc.) would clearly be useful, but that is the subject of a different list: Major Branches of Major World Religions.
This list is based primarily on the degree of doctrinal/theological similarity among all the various sub-groups which belong to these classifications, and to a lesser extent based on diversity in practice, ritual and organization.
It used to be the case that there were various list entries titled list of notable boozers, list of famous seventh day adventists, list of celebrity Americans, frequentists, famous flamers, [list of IRV advocates]].
I checked on wikipedia talk:naming conventions and decided to rename them all list of X for consistency, findabiliy, linkability, etc. If you were fine with list of notable Jews, you should be fine with list of Jews - it's just a name change to fit in with conventions, not a dramatic repositioning in the market.
This can all be easily handled by appropriate introductions - list of popes can say "this is a comlete list of..." and list of atheists can say "this is a partial list of atheists, focusing on atheists who are particularly notable, famous, or important".