And includes presumed future increases of money supply. So really it's the fiat currencies that are volatile, though the governments producing those currencies are in collusion so it appears they aren't volatile relative each other.
BTC is volatile in large degree due to the inflation (in the true sense of the word - increased money supply) of fiat currencies. It's finding its place so there is some true volatility as well. And governments can obviously manipulate prices so there's that.