POST Titles in bold

Why when I view a forum are some posts in bold and others not? I see no Pattern

Bold, or highlighted as I describe them, are threads that contain posts you haven't read yet. Once you open them and read the most recent post then it won't be bold anymore, ideally. Occasionally I will notice that it's still bold after I just read it but sometimes that's because I hit the back button on my browser instead of clicking one of the menu links on the top of the page.