I can't reproduce on your site currently.

If you can leave an "unread" topic there that actually was read, so I can go and run some database checks, I will take another look.

Right now I suspect one of the following cases:
1) Explicitly invited into old topic by someone
2) Topic accidentally marked unread
3) System clock changed
4) "Post view-history memory" config option increased, leading to a gap of old "unread" topics that may have been read but have not had the details of that maintained due to the prior lower version of the option leading the tracking to being already erased for that period (anything older than "Post view-history memory" is assumed read)
5) Someone edited a forum post within the topic, which would flag it unread
6) Someone really did post in an old topic
7) Some other admin using 'SU' then marking stuff unread