California includes your total income, rather than just your California income, on a nonresident return because of the way California calculates tax.
The state starts with federal adjusted gross income and then subtracts your federal itemized deductions to come up with California taxable income (after some state-specific adjustments).
This is the base tax, or the amount of tax if all your income was earned in CA. You are then taxed on the CA percentage of this income. So if 10% of your total income is from CA, you pay 10% of the base tax.
You can see the calculation on lines 31-42 of Form 540NR for 2023. California doesn't have the 2024 Form 540 NR posted yet on their website.
More information about filing nonresident state returns is found in this TurboTax help article.
@Sedes Gobhani
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