The gap between gross pay and take-home pay is produced by a calculation the employer performs each pay period. Understanding it explains why one household gets a refund and another owes.
Withholding is an estimate of annual liability
An employer does not know an employee's total tax situation. It has one input, the withholding form on file, and it projects annual income by annualizing the current paycheck.
From that projection it computes an estimated annual tax, then divides by the number of pay periods to determine what to withhold now. Each paycheck is a fraction of a forecast.
Because the forecast assumes the current pay rate continues all year, anything irregular distorts it. A raise, a bonus or a mid-year job change all break the assumption.
The form drives the entire calculation
The employee's withholding certificate tells the employer about filing status, other income, expected deductions and dependent-related credits. Those entries adjust the projection up or down.
Older versions of the form used allowances, while the current design asks for dollar amounts directly. Employees who have not updated their form in years may be working from outdated logic.
Nothing in the process validates the entries. The employer applies what it receives, so an incorrect form produces incorrect withholding all year without anyone noticing.
Multiple jobs break the arithmetic
Each employer calculates as though its paycheck is the household's only income, so each applies the lower brackets and the standard deduction independently.
Two jobs each withholding as if they were the only one will collectively withhold too little, because the combined income sits in higher brackets than either projection assumed.
The current form includes a section specifically for this situation, and households with two earners commonly encounter the shortfall when they do not complete it.
Supplemental pay follows separate rules
Bonuses, commissions and certain other payments can be withheld at a flat supplemental rate rather than through the regular projection, which is why a bonus often appears heavily taxed.
That perception is usually about withholding rather than about tax owed. The flat rate is a collection method, and the actual liability is settled when the return is filed.
An employer may instead aggregate the bonus with regular wages, which produces a different withholding figure for the same payment depending on the method chosen.
Reconciliation happens at filing
The annual return compares total tax owed against total withheld and payments made, producing either a refund or a balance due. Withholding is a prepayment, not the tax itself.
A large refund means more was advanced during the year than required, while a large balance may bring underpayment consequences depending on individual circumstances.
Rules, rates and form design change between years, so anyone with a materially changed situation is better served by a tax professional than by carrying last year's settings forward.