Derpi's custom-made forum software has a peculiar quirk when it comes to the "last post" column: it continues to show the name of the thread with the last post, even after that post has been deleted. So when a spammer makes a new thread and it gets deleted, the thread will still be visible on the index page (even if it cannot be accessed anymore). I **hate** that, so I sometimes make dummy posts just to kick the spam thread out. Fast-moving forums, like General Discussion and Generals, don't suffer from it much, but on a slow-moving forum, like Uploader Discussion, the spam thread could potentially be visible for days, even weeks unless someone else posts something.
Why do I post in your thread then? I don't know, I just needed some "normal"-looking thread to post in. I'll try to pick some other thread the next time this happens.
(EDIT: I'm spamming your thread... to get rid of someone else's spam. I didn't think about it that way until now.)