Hi Nicky-
The W4 has really changed so you really don't claim "0 or 1" anymore. The new way your withholding is calculated is to take each paycheck and multiply it to make it annual pay. Then they figure the tax on the annual pay (less standard deduction and based on your filing status) and then they divide the annual tax back down to a per paycheck amount. The dependents comes down if they also account for potential child tax credits on what you owe.
If you are starting a job mid-year and already had income that can also skew what you are looking for.
