We know that Polaris is situated NEAR to the north pole and NOT exactly on the north pole. Polaris is always located within 1 degree of the North Pole.
Determining Latitude by polaris involves the use of GHA Aries which is measured with references to the CELESTIAL POLE. Since Polaris is not exactly located on the North Pole, 1 degree is subtracted while calculating the latitude from the polaris tables to adjust for the assumption which is taken in to account that polaris lies on North pole.