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.