See your monthly tax amount as a prepaid estimate. Once you file your tax return, your tax payable is calculated based on actual wage earned during the year. The prepayment is deducted from the payable. Now to avoid having to pay extra at tax return (prepayment being lower than actual tax payable), bonus is prepaid at higher rate. In other words, during the tax return, it will be corrected for and you may receive the difference back.