Because you're comparing the value of one thing over time against the rate at which you would gain value in a different investment over time. I can see an argument for why you might say tbonds have been a better investment than gold, per se, but that's not what I'm talking about. I'm talking about the necessary volatility of fiat money because it's not backed by anything of real, unchanging value. I'm talking about the purchasing power of the same amount of gold and USD over time.