I tried explaining once, but I erased it. I think an example would be better.
I go to wwwthreads.pl and see that one thread has 2 new messages. I go to that thread. I see that two sub-threads each have 1 new message. (Keeping it simple.) I go to the first thread and read the new message. Then I *don't read* the second new thread and go back to wwwthreads.pl. From here I do not see an alert that there is still 1 new message. But if I go to that thread again, the sub-thread that I didn't read still shows 1 new message.
Anyone else notice this too?
