It's because only those posts that a
member has never opened, or has not opened since something new was added to the thread, will appear in the New Posts list for any particular
member. As Dan said, since you had the thread "open" while you were typing it, it won't appear on your New Posts list until someone else adds a post to the thread. Once Dan added his response to your post, the thread went back onto your New Posts list.
To see a complete list of posts, whether you've opened any of them or not, click on Quick Links, then Today's Posts in the drop-down menu.