Using the Back button on your browser causes it to display the copy of the web page it has in it's cache. The PHP and/or Javascript code behind the page does not run again.
When you do as JB suggested...
Typically, once you're done reading a thread if you will scroll to top of page and click the forum instead of just clicking the back button on your browser it will mark a thread as read.
Originally Posted by johnnybax
You cause the browser to get a new copy of the web page from the site's server. This also causes the PHP and/or Javascript code behind the page to execute again, updating the information the web page displays.