Although I am not going into detail since this might be a bit technical, the inflation rate based on the CPI that the Bank refers to tends to be higher than actual inflation. Given this, aiming at zero inflation in terms of the CPI means aiming at negative inflation, in effect.
Therefore, in order to ensure price stability, the inflation rate should be sufficiently positive. The next point regards securing policy room for the future. The level of nominal interest rates is usually determined in accordance with that of inflation rates.