You write as if withholding was an error in calculation. Maybe it is, don't know how it works in the US, but here withholding follows a different, very simple

You write as if withholding was an error in calculation. Maybe it is, don't know how it works in the US, but here withholding follows a different, very simple formula, since it's impossible to know the real amount ahead of time anyway. I disagree that the IRS has an incentive to maximize taxes. They have an incentive not to lose time with disputes. But even if you don't trust them, so what? You can still check the numbers yourself, and file your own if you disagree.