And that's a problem, isn't it? I shouldn't have to be immersed in a culture to use the system. You've traded usability and user experience for ... a cultural in-joke. "Hi, this is AO3, and our tags aren't anything like anyone else's tags, but we're still gonna call them tags" is a problem. It's like if I made a search bar and it returned random results. It says search, but culturally, we give you random results. That's how we do it.
But it's not a problem because it doesn't affect your ability to search. One-off tags do not enter the tag search results. I'm super confused why this isn't obvious and intuitive to you..
That's why we developed librarianship.