Sometimes they have set time allowable for certain repairs that is automatically calculated. In addition, unless you witnessed the repair, they may have had more than one individual working on the car at the same time.
To protect yourself, you should have protested the amount of time that was charged at the time you received the car if you were unhappy with the charges.
Please note that this response is based upon the limited information available in the question. In addition, it is not intended to create an attorney/client relationship and is offered only as general information and not as legal advice.