I noticed when you are on the forum home page, and click the Mark All Forums Read, all forums will be marked as read with the exception of those with last post times of "A Moment Ago". Is this by design?
It's due to the posts being submitted at the time I pressed the link. It just seemed odd that there were always ones left unmarked, but Brandon_R nailed the reason. I did some testing afterwards and those that had the "A Moment Ago" did get marked as expected.