Ah! It's because we had a backlog of unapproved potential members. A few days ago I went through and cleared it out, which is where the May new member account came from. It had been sitting in the list since May. I checked and found that even if the IP had been a spammer then, it was long since stale and no recent spam activity had been reported, so what the hell, why not approve.
Now why the backlog got so bad, is the fault of myself and the other admins not keeping up with it.