Why might two people in the same state with the same salary pay different rates of income tax?

One possible answer is:

They could have had extra money withheld when they completed their W-4.

Explanation:

When you complete the W-4, there is a worksheet that you fill out to help you decide what number of exemptions to claim. However, you can choose to have more money withheld, to ensure you have no tax obligations at the end of the year. This would cause the amount of money withheld from two different people to potentially be different.

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