Find out why it is different first, as there are some state such as NJ that do not allow per-taxed medical premiums to reduce the taxable wages. First I want to make sure the W-2 is correct.--This is starting point.
Generally when you have two part-year state returns you only pay tax on income earned in each state and there is no credit for taxes paid in another state.
Your situation is not in the general category, however what you will have is some double taxes income, so the CA return there will be credit for the taxes from MA on the amount that was taxed by MA and CA for that one month of income.
