An employee is responsible for how much taxes are withheld from his check because the withholding largely depends on the information he supplies his employer. Depending on certain circumstances, his federal withholding may change with each paycheck.
Your federal taxes withheld from each paycheck should be based on your most recent W-4 information.
Your allowances, filing status, income and the IRS withholding tables are also used to calculate your federal tax withholding.
Generally, the more allowances you claim on your W-4, the less federal taxes are withheld from your paycheck. Each time you change your W-4 to reflect more allowances, your paycheck reflects less taxes.
If you are paid commission plus a salary, and the commission gets processed as normal pay, it will place you in a higher tax bracket, changing your federal withholding.
If you are an hourly employee and your hours for each pay period are different, the amount of tax withheld varies, because your taxes withheld also depend on your income.