"unread posts since last visit" will give you a snapshot, once, of threads you have not read yet. This is a one time thing. The "visit" is the clicking on that link. Once you do that, that is considered a "visit" so next time you log in you will not see any of those threads under "unread since LAST VISIT". If you don't have time to read them all to truly have zero unread threads, you use the "all UNREAD topics" and will still see all unread topics until you read them. You can keep coming back to that link and they will stay there until you have read them.
Hope that helps!
-Shawn
