I just feel like reporting what happened and maybe it is not only me who is experiencing that in the unread posts lit. It really isn't a big deal but I think it's a bug in the forum.
After all, it seems logical enough: 337 posts ranges ~8.4 pages given that they have all been in distinct threads, and I'm assuming you've checked some updated ones previously.
A minimum of 58 posts have to have been posted in the same thread, or 58 threads would need to have been checked, or any combination of the two.
* hardcap of 6*40 + 39 = 279 posts ranging 6 pages; 337-279 = 58