The forum marks new posts since your last visit to the forum, not actually how many you have read or not. If you read a thread, then the link to the thread is no longer in bold, but it isn't just registering what you've read.
Your session expires after a certain length of inactivity. Once a new session begins, it will show just the threads with new posts since your last session ended.
This is built into the forum software, and it cannot be adjusted to keep every unread thread in bold.
I hope that helps explain it
Unread Posts Refreshed after Inactivity
