Well, the reason people refer to non-franchise characters as 'original characters' are more complicated.

Way back when, in the early days of the internet. everyone was very careful to place copyright information on any fan art or fanfic they did, because people were actually, literally getting threatened with legal action. When 'everyone' does something, it's easy to assume you need to do it too. "everyone says 'Stuff like "Lestat belongs to Anne Rice, but Ebony Dementia Raven belongs to ME!' ... so maybe I need to clarify that it's mine?"

Plus, when you create an interesting character, there's a lot of people who'll ask "whoa, he looks cool, what's he from?" leading to the explanation of "no, he's mine, I made him" and it's easier to forstall that by just explaining that they're an original character, not from any franchise.

Or that they're a fan character in the such and such setting.

Obviously, we're no longer in the dark days of the itnernet, but some habits live on