There's your problem Martin - you're using Internet Explorer!!
I think it's to do with cookies and security/permissions. The last accessed date/time is stored in a cookie that's saved on your machine, so when you come back it knows how to calculate the difference (in post counts) between this login and the last one.
The problem seems to be, and this affects/affected iPad/iPhone (Safari) browsers as well, is that the website sometimes doesn't/isn't able to read that cookie and so it gets the last time you visited wrong and can't figure it out properly...
I've searched to see if there's something we can do to fix this but it seems to be an inherent problem in the version of the software we use, and I'm not keen to go looking through the code to try and hack it as I'll probably do more damage!!
I really want to get rid of the "Today's Posts" feature as it makes the website look less used than it really is. This is why I've put the "What's New" feature back in.
"Today's Posts" will only show the threads that have been created or had new posts put into them in the last 24 hours.
"What's New" will show you the same as "Today's Posts" but it will also show you a list of anything that's NOT been updated since your last visit but still contain unread posts - ie, things you've not looked at.
As an example, the posting as of right now are:
Today's Posts: 9 threads
What's New: 0 new threads and 16 unread threads
I think moving to the latter will help to show that the site IS being used. It's also the same method used by most websites when showing unread threads/posts.
The "Today's Posts" feature was great when we were getting 50-100 posts per day, but now that we're running a lot lower than that it's misleading!