Thanks for the responses
And yes, it happens after reading only one or two posts on a page. I've never noticed it before.
I've noticed it particularly because I'm watching my NHK Trophy recordings of the competitions atm and following pbp on here. After reading a few posts eg on page 4, then check running order on page 1, I'm taken back to page 5 even though there are unread posts on page 4. (If you repeat these actions, you then go to page 6 even if no posts were actually read on page 5!) It's just odd.
** ETA clarification