Sometimes if you don’t scroll all the way to the bottom of a post, it’s not counted as read - right now, I can see the little rectangle telling me who made the thread and who replied and so on. I can also read all of Stephie’s post but the counter to the right of the screen is saying I’ve only read one post in this thread. As soon as I scroll far down enough to get past Stephie’s username, it counts the post as read.
If it says you have unread threads, I’ve found it to be a couple of reasons - one, I have a another CAF page open where I’ve clicked on the circle and it’s told me what posts I haven’t read, but I’ve not closed it so it doesn’t come up anywhere else that I have unread posts. Two, if you click on the circle once, you might have 3 posts that you haven’t read, but opening it means you’ve seen the notification so it goes away.
I’ve explained that awfully (where’s the blushing emoji when I need it??), but that’s what I’ve found. I hope I’ve helped a bit!