Basically because the velocity of money is variable and depends on societal factors (lending practices, payment technologies, even psychology). What the central

Basically because the velocity of money is variable and depends on societal factors (lending practices, payment technologies, even psychology). What the central bank is really looking to do is keep the amount of money that's in flight supporting transactions at any given moment roughly stable. When velocity drops the money supply needs to increase faster than inflation to keep the amount of cash in flight the same. https://fred.stlouisfed.org/series/M1V https://fred.stlouisfed.org/series/M1V