How very odd. And you know what's even odder? I checked out your account using IE and Firefox, expecting to get the same results, however, Firefox comes up with three times more unread posts than IE does.
I have no idea why that's happening. And it doesn't just happen for you - it happens for me too. However, unlike you, when I click on the "All unread posts" link, the missing posts do show up in IE.
Something very odd is going on. And that leads me to warn everyone that using the "Show unread posts since last visit" may not be reliable at the moment and could be missing posts. A better alternative might be to use the "View the most recent posts on the forum" link down in the "Forum Stats" section of the "Info Center" on the BoardIndex page (the main drawback of which is that it brings the latest posts up first, which means you can easily read replies before you come across the post being replied to). Or to just visit each board individually whenever it displays the "New Posts" icon (which is what I've always done) because it's apparently the most reliable method for catching everything.
And when I get a chance, I'll have to check out the SMF home site to see if anyone else has been experiencing this same problems with "Show unread posts since last visit"...