I have noticed on several occasions, like today for instance, that even if I perform the "mark Forums Read", (which I performed yesterday) option "all", today the threads were not be flagged as read.
Just tested here by doing the Mark All Forums Read, then clicked on New Posts again, and no threads come up, since I just marked them all read. I'll look at them again in a little while and see if any that come up as New Posts, actually do not have any new posts in them.