i believe that depending on your isp/browser/parent's shoe colour etc, the browser often just re-paints the screen, or brings down information that is new from the last time the page was loaded... it doesn't always force the entire page from the server, much, if not all info still comes from the cache... CTRL guarentees that the page is refreshed from the server....
i think that's what it is at least... i'm not an OS/IT person, maybe someone else can chime in... but that's my understanding of it...