It is because the National Health Insurance premium is calculated with different formulas depending on the municipality.
The National Health Insurance program is managed by each municipality with the premium paid by the participants and disbursements from the national government.  The number of participants, age composition, medical costs to be covered by the National Health Insurance program differ in each municipality, and consequently, the insurance premium required to manage the program is different from city to city.

Please inquire at the municipal/ward office of your residence for details.
◆ For residents in Osaka City.