Tony, do you close your web-browser between visits to the site?
I think that unless you do that the site thinks that you have not left it - so from that point of view there are no "new" posts for you as you have never been away.
Also, if you log-in to the site, then close your browser, then immediately open it and log-in again, then any topics which may have shown as "unread" when you first logged in will no longer be listed when you log-in the second time even though you did not read them on the first log-in, because from the point of view of the site there have been no "new" posts since your last log-in 30 seconds ago.
Does any of that make sense
Never was very good at explaining things.