This is a known issue, poll votes trigger the "new unread posts" indicator. There are, in fact, no other replies. See this CSI thread.
That said, I did notice some issues as well, the blue star sometimes doesn't disappear after having been bumped by a poll vote, even after opening the thread. It seems to have resolved itself, though, a few days ago the "So, Windows 8" topic was permanently marked unread no matter what I did.