How is federal tax withholding calculated per paycheck? (opens in new tab)

I just started at a new company and received my first paycheck. It included a signing bonus, which of course made the paycheck much higher than a normal one will be. The fed withholding amount on the paycheck was much higher than I thought it would be, by a few percentage points. How does payroll normally calculate the withholding amount? Obviously, the amount for the whole year can't be calculated, so it has to be predicted in a sense. But does payroll use the amount that they know is my ann...