Unless you log out of the site, or your session expires after you have not exchanged information with the server for more than 30 minutes, the "jump to newest post since you last visited" link will take you to whatever was the newest post when your current session started, even if more posts have been made during your session. Does that make sense? I have also found this to be quite annoying, especially in a popular thread, and I do believe it has been fixed for the next version of vBulletin.