The difference between the growth rate of monetary base and that of money stock can be interpreted in the following. The monetary base consists of all banknotes and all current deposits that banks and other financial intermediaries hold at the Bank of Japan.
A textbook of monetary economics tells you that the Bank of Japan supplies the monetary base, on which basis banks extend credit so that the monetary base creates a multiple increase in the money stock. In the course of discussions about monetary policy at present, an argument is sometimes made as if the monetary base were in shortage.