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Why ADP does not accurately withhold state and federal income tax (even if W4 is correct)?
I see several interesting statement in your question.  A. my only income is from my Employer B. I also receive employer stock (ESPP, RSU, NSO). However, employer withholds taxes for these stock transactions through my broker (I see them broken down on my W2).  C. I have been subject to Alternative Minimum Tax. A implies a simple tax return. B and C tell the opposite story. In fact if B is not done correctly The amount withheld due to payroll may be perfect but the under withholding could be due to the ESPP's, RSUs and NSOs.  The AMT can throw everything else out the window.  If a person has a very simple tax situation: Income doesn't change a lot from paycheck to paycheck; they take the standard deduction; the number of exemptions equals the number of people in the family. Then the withholding is very close to perfect.  The role of the exemptions on the W-4 is to compensate for situations that go above the standard deduction. The role of extra withholding is when the situation requires more withholding due to situations that will bring in extra income or if the AMT is involved.