This says that if the money supply increases by 5%, in the same period we should have the same increase in the real output to have stability of the price level or of the purchasing power. But…
Real output grows for reasons such as technology and capital available to workers and so this growth is related not only to the money supply.
Then we can say that if real output increases only by 2% while money supply rises by 5%, this means prices grow by 3% and consequently the inflation rate is 3%