Kinda. The number isn't made up, it's based on your salary from that job and then a calculation based on the number of dependents you will claim (which generally lowers your tax liability so they withhold less).
At the end of the year, this tax filing process is to reconcile if the amount you had withheld matches with what you owe the government.
At the end of the year, this tax filing process is to reconcile if the amount you had withheld matches with what you owe the government.