for some websites the pages are reloaded, for ads. probably the longer you leave the page idle, as it refreshes more and more times, for newer ads, the more youll have to press back.
there should be a dropdown arrow to the right of the back (and forward) buttons. try clicking that little arrow, and it should list the most recents websites.. and itll probably list the current page afew (3, in your case). one way to stop this is to block the ip/website providing the ads