Open letters, closed letters?

Recently, I spent 3 weeks away from my computer, with limited access to a really slow one. I wasn't able to keep up, and looked forward to coming home and catching up and learning new SVX and non-SVX thing here. I knew that all I had to do was go to a thread, look for the open envelopes, and hit the icon to read the most recent unread post. Imagine my surprise when I was posts dated during my time away on closed envelopes. This made it a little harder to see what was new. Is this normal? Could it be because I had already received an email telling me there was new stuff here? It's not critical, I am just curious, as I have to spend another couple of weeks in OK soon, and am wondering if there is a way around this. Thanks guys, this isn't criticism, it's a very user friendly site after you use it a while.
Mike