If you are a dodgy person not wanting to be traced on the web you can get IP masking tools, these tools mask your IP address and will change your IP address contiously.
Active users lists are nevr acurate, they are only ever a guide. Look at any live stats program that tells you the number of users on your site, or any active users program, none are acurate.
HTTP, the protocol for looking at web pages, is a stateless protocol, meaning that for each page, image on that page, etc. a new connection is made to the web server, once the page, image, etc. is downloaded the connection is dropped to the web server and there is noway of telling if that user is just reading the page or closed there browser. The next page that person calls the web server has no-way of know if this is a new user or the same user again. You can use cookies to keep track of users to see if it is the same user, but then if the user has cookies disabled there is no-way to track the user.
The active users list for the forum could be changed to also check the username as well, but as the code for the active user has to be fired up each time a page is clicked on this will also effect performance of the forum engine and also add extra overhead on the web server. As the active users list is only for a rugh guide and not essential in the running of the forum I can't see the point in the extra overhead.