Precious Metals trade in the interbank market, very similar to fiat currencies. There are different locations where Precious Metals can be settled, e.g. London, Zurich, etc. and the price might vary depending on location.
Revolut is trading in the London interbank market for Precious Metals. The price reflects the market value of 1 troy ounce (a standard unit of measure for precious metals. It is roughly equal to 31 grams).