Remember: 20,000 accounts, not members. Like LiveJournal, nothing prevents a person from signing up a dozen sockpuppets or alternate accounts. (Unlike LJ, there's not a lot of reason to do so; I don't imagine there are many people with a different account for each of their fic fandoms. But there may be some with different names for the fic originally stored at different archives, or preventing variations on their names from being used.)