> I just clicked the "Reply" button on the very bottom of the page, who
> knows what kind of magic went on.

That may be a bug in the forum software. Normally one would expect to reply to the thread opening message when clicking on "Post Reply" at the bottom, not to the thread's most recent message (which is really just a random message in the context of a thread). As long as this potential bug exists it's probably best to avoid the "Post Reply" function and use the messages's own "Reply" function instead like you just did.