From my understanding:
New posts are posts you haven't opened.
As soon as you open a thread and then you open another one the number decreases and the thread is no more visible in the New posts list. For me t
If you click on Recent posts all are displayed.
And for me this is as it should work, you see only the threads with new posts not yet opened.
The number besides New posts is the number of posts and not the number of threads.
If there are 3 new posts in a thread, the number shows +3 but only the thread is shown in the list.