I need help explaining the calculation of FIT withholding on an executive bonus payment.

We paid executive bonuses in January with an element that is set to process separately. I thought because it was set as supplemental payment and processing separately that it would deduct 22% FIT withholding but the calculation was much more than 22%. I recalculated the FIT withholding using both the bonus and regular pay amounts and was still not able to come up with the same FIT withholding amount that was deducted. My manual calculations are about $1,000 less than what the system calculated. I need to be able to explain this calculation to the team member.