When viewing a thread you have previously visited the first post you see should be the first new post since the last time you visited the thread. This saves having to scroll through pages of posts in popular threads to find where the unread posts begin.

If you find that you are being taken to the last post in the thread for both old and new threads - the problem may be that you are clicking on the "last post" link instead of the "subject" link when you click on a thread.