Differences between fandom and non-fandom based RPGs?

Ok, you guys have "Fandom-based RPGs" and "Original RPGs"... which would an RPG based on a current role-playing game system (like Dungeons and Dragons, Werewolf, Vampire, Mage, Exalted, etc) be? Technically, those who play such games are fans of them, which would make them fandom-based. At the same time, I am not sure what you all define as "fandoms", and as the stories (and characters) themselves are original, it might fall in the latter. An explanation (or your thoughts) would be appreciated. Thanks!