4Sword, if it's me, why would it display as a guest?
Because you are displayed elsewhere with an updated action. Really, I know what I am talking about, I have seen members with their action indicated as posting in a topic, then seeing them either viewing that topic or returning to the board, and then seeing a guest's action be that the guest is posting in that topic.
But yeah, what Q.K. said, really. And by replicating the effect as I mentioned earlier, if you have ever created a small forum and then registered on it, you'll see peculiar guests acting the same as you; and in all circumstantial likelihood, they are you. That effect doesn't seem to appear as much on more active forums due to the occurrence getting "buried" under other user actions; and possibly better hosting on more active forums being able to process information better.