I don't understand what you mean by "supposed to be correct." It means that if your actual income and withholding for the year turn out to be exactly what you entered in the Tax Withholding Estimator, then on your tax return at the end of the year you will have no refund and no payment due. Your total withholding will be exactly equal to your tax for the year.
That, of course, is an estimate based on the information that you entered. Don't count on it coming out exactly that way. But it should be close. You should have a small refund or a small payment due. Of course, any difference between the actual figures for the year and what you entered in the Tax Withholding Estimator will change the results.
It's an estimate, not a precise calculation.
