When I hover over it, it says "You have founded a religion". What confuses me is how it can vary. It only mentions founding the religion, it doesn't seem to take into account how successful the religion is, or if it does it gives no indication.
Mysteries within mysteries lol
Probably how successful those religions are relative to the other religions on the continent. "Measured by followers through time" would be a likely metric, given how most of the other fame values go.