Doesn't matter what you are actually going to make. The payroll program is only smart enough to assume you make the same every paycheck.
if you make 3 paychecks a year:
1: $100 - program assumes you will make 300 per year, no tax
2: $20000 - program assumes you will make 60,000 per year and takes money from higher brackets from this paycheck.
3: $100 - program assumes you will make 300 per year, no tax again.
At the end of the year when you do your taxes, you get all the stuff from higher brackets back.