I understand that option as this: it provides a list of all the threads that have new posts added since the last time I was on the forum, not just the threads you have posted in or brand new threads. For example, when I close the forum and head for bed my current visit is over. When I log back in in the morning, the "Show unread posts since last visit" option gives me a list of all the threads people posted in while I was sleeping and off the forum. All the activity that happened while I was away, essentially. It talks about "unread posts" not unread threads.
Does that make sense, Leto?