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The problem with that line of reasoning is that HTTP versions aren’t black and white like this; just because something advertises itself as HTTP/1.0, doesn’t mean it doesn’t understand HTTP/1.1 (see RFC2145 for more).
If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer expiration time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache.