Answer:
The IRS considers that every single salary paid is a distinct and unique event for taxation purposes. The employer is supposed to estimate the annual salary. But if this cannot be determined because the payroll is not regular, then every time a salary is paid, the employer should deduct an estimated amount depending on the wage-bracket tables published by the IRS. This is not exact, but the employer is responsible for doing this. If the employer withholds too much money, it will be refunded. Or if the employer doesn't withhold enough money, the employee will pay the difference.