That's called a progressive tax. You get paid more, so you're estimated to be in a higher tax bracket. Payroll deductions "per check" are estimated as though that is the same paycheck you will receive for every pay period of the year. The fact that it is a temporary job is not taken into account. This is why no hourly wage worker ever has a perfectly even tax return at the end of the year. There's always a little additional tax owed or a refund.