Probably something to do with your browsers cache. Maybe you have it set up to automatically empty or it doesn't accept cookies so the site has no way of recording what you have or haven't read.
I don't think that is it. When i go to my other computer it remembers that i what i have read. I think what is the problem is that you need to click mark as read at the top. It only gets rid of the new if you do that.
I would rather live my life as if there is a God and die to find out there isn't, than live my life as if there isn't and die to find out there is.
Albert Camus
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Thanks,
Parrothead