The formula ignores "Protocol controlled RUSD" because any RUSD that the protocol holds will never be sold for PCV, only burned. Protocol controlled RUSD can have second-order, short-term inflationary effects. For instance, RUSD deposited into a lending market by Ring Protocol could increase the circulating supply when borrowed. The interest accrued and eventual withdrawal of that RUSD ultimately have a net deflationary effect in the long term.