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Taxes can be variable for a variety of reasons, including how many dependents you claim. That usually controls how much is withheld from your income. Otherwise, someone could just claim more dependents to have more money withheld from their income, thereby reducing their child support, but then get a big tax refund at the end of the year. The program is estimating what your taxes should be if you get no refund and owe not taxes at the end of the year. It cannot be precise, though, because each person's tax situation is so different. For example, do you take the standard deduction or do you itemize? That will change your actual net income as well.
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