I think the reality is "more total atoms" and "less gold atoms" per unit of volume. Carat (for gold) is a ratio of mass. If you mix gold with a lighter element

I think the reality is "more total atoms" and "less gold atoms" per unit of volume. Carat (for gold) is a ratio of mass. If you mix gold with a lighter element than usual you can put a bigger volume of the light element. That what people tell when there is "less gold". The light atom could be more packed if they have stronger chemical bonds, I think it's the case for ceramics. So there is more atoms (gold + others) per unit of volume.