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Also Welcome to the board [webmasterworld.com]
But yes it's a combination of inbound links and content changing.
It's easier to get a "fresh tag" if you content change often.
And it's easier to get the more inbound links you have.
And no all sites don't get the "fresh tag", it used to be around 3 million, now it's a lot more.
Also a interesting thread is Are you using If Modified Since? [webmasterworld.com]