I've noticed a lot of sites exhibiting these faulty semantics recently. For instance, twitter.com seems to have different content almost any time I refresh. Me and a colleague did a quick test, submitting a GET to Twitter at the same exact time. Her screen showed a completely different set of content. Strange and troubling.

your expectation is fundamentally flawed for many reasons, but I'll stick with the most general: the Internet has no HTTP police.

Fortunately there are no HTTP requests going on that cause this behavior. It is done by JS, using Local Storage.