The withholding system does annualize your income, so if it "thinks" your annual income is 29120, it'll use the Head of Household standard deduction amount to figure your taxable income. But then if you said you have two or three kids that you'll be claiming, perhaps you entered a $ amount related to tax credit for qualifying dependent children. Your tax is less than the sum of those tax credits, so your actual tax liability is 0, so no withholding is happening, and since the child tax credit is a refundable credit, you may actually receive money.