It depends, but is likely how that state determines tax.  A number of states use a similar method to calculate tax on nonresident income.  The state will pretend that all of your income is taxable in that state, calculate tax as if you were a full-year resident of that state, and then prorate your tax to the amount of income actually earned in that state.  They will only tax the income earned in the state, but it will look on the return as if all of the income is being taxed there.  But if you look carefully on the return, you should be able to see that a prorate has been calculated on the tax.