https://en.m.wikipedia.org/wiki/Progressive_tax
The withholding is calculated by extrapolating how much you'd owe in taxes if you earned this amount for every paycheck. If you know how much you're projected to earn over the year, you could ask whomever handles your pay to adjust it to the correct percentage. Either way, come tax time it'll be settled and you'll either get back however much you paid in excess, or you'll have to pay a bit extra at the end of the year.