Withholding is not your final tax
The optional 22% rate applies only when its conditions are met. Supplemental wages above the annual $1 million threshold are withheld at 37%. The aggregate method combines regular wages and the eligible bonus portion, then subtracts regular withholding. Both methods also account for incremental FICA.
Source: IRS Publication 15, section 7. This calculator does not handle multiple supplemental payments in the same period beyond the entered combined bonus, benefit changes or special W-4 adjustments.