I would imagine that it depends upon the thread's position at the time of posting. If posts are made to the other threads that are above that has your interest while you are composing your response, then it is possible that the position in the list might not change. One other possibility is that you might be using the "back" button of your browser to view the thread list.

If it isn't due to either of these reasons, then my guess is that it is just you.