Not "always", but the flat rate is the way EDV chooses to do it. If they added it to your pay and withheld it at the lower rate, it would increase your income, and you would have more withheld monthly than you do now, since it would put one in a higher tax margin.
Originally Posted by
gojo
And why wouldn't they expect that already? Bonus tax is always withheld at a higher rate. That's why many elect to have it put in their 401k