Marking Forum "Read"

I can read the new posts and when I come back later, old posts (i.e. the old threads) I have read show up in bold again as threads with new posts.  Even when I click the "Mark Forum Read," I come back after closing my browser and old posts/old threads show up bold again like there is new info.  What's up with that?  Am I doing something wrong?
 

I think that is a cookie issue....  if you log off versus just closing the browser it should not be bold....