It's just not making sense to me, and if I don't get it, there's no way I can explain it to my members.
It IMHO really isn't thaat difficult to understand:
What's New shows the 10 threads with the newest replies, no matter if the visitor already read them or not.
New Posts shows threads with unread posts, no matter if those are the threads with the latest posts or not.
(With dedault settings, if settings are changed things could be different)
I understand that the "What's New" is limited to 10 by default. So if I have 20 unread posts only 10 will show on "What's New" and all 20 will show on "New Posts".
If you have 20 threads with unread posts, up to
10 of them might appear in the widget.
But it could be that none of your unread threads does appear there if all your unread threads have last replies that are order than the 10 threads shown by the widget.
But in my case zero are showing on "What's New" and 20+ are showing on "New Posts".
Can you post a screenshot if the configuration of your widget?
What is the definition of "recently"?
There is no explicit cut-off, the widget does show the 10 threads with most recent/new/latest replies that are newer than the read marking data lifetime (default = 30 days).
So even if there had been no new posts for hours or even a few days, it would just show the latest 10 (or less as on my screenshot if there are less than 10 threads in total, which is the case on this installation)