I think a lot of people join any new forum they find automatically in case it has hidden content they cannot see without joining. Others may join meaning to post, then decide it isn't worth it, or to ask a question they suddenly realise they know the answer to. Others will be botched attempts at joining where they put details in they didn't mean to and would prefer to change, or leave something important out.
So there are many possible reasons, as you see. And I'm sure there's a load I've missed <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
