Eh, the misunderstanding what New Threads actually are is not new. By logic, New Threads are just, newly created unread threads, not read threads with new unread replies. "What's New" has a wider scope than New Threads, because it includes actual New Threads and Old threads with unread replies. So to summarize it:
New Threads = New Threads
What's New? = New Threads + Old Threads with unread replies.