Sounds to have something to do with caching of the page in the browser. As the userlist contains dynamic data, it is "normal" that you get an expire message, as it should be reloaded to get the context.
Normal back button on a browser just "moves" on page back in its cache but does not nessecarly do a reload of the page. If i remember right joomlas back button in a page does the same triggered through javascript.