I'm not entirely certain what causes this, but sometimes, the latest unread post marker will be a page past what I've actually read, so when I pick click on a thread from my bookmarks, it will skip a page.
I think possibly it might have to do with browsing on 2 different devices simultaneously?
For instance, just now I went to my latest unread post in the D&D election thread on page 28, which is correct. There is a 29th page as well, though I haven't navigated to it.
I then hit the bookmarks popup on my mobile device, and it now shows no unread posts in that thread. I didn't browse to the thread on that device, but could it somehow still be causing it to update the pointer?