It's because I've deployed various spam detection measures; some get through sign up as humans and allow their spambot automation to take place, and then either Akismet or StopForumSpam takes over. There is another measure, which adds hidden fields that only spambots can see because they are set to fill any form fields out on a sign up form, regardless if it's hidden or not. If they fill all of the dummy fields out, they get put in the Gulag.