Regardless if it's an overtly dishonest attempt to pocket unused supplies or an honest oversight, many homeowners don't get everything they paid for when their contractor says the job is done.
Unless you're watching over him like a prison warden, there'll be no way to tell if you're getting the right amount of insulation -- it's easy to make the stuff look abundant, even when there's not much there -- or if he used enough expansion joints (which allow cement to expand and contract without cracking) when he paved your driveway. By the time you figure out what you're missing, your contractor and your money will be long gone.
One way to prevent any accidental or intentional oversights is to insist on a detailed line-item contract, which provides a line by line breakdown of the necessary supplies and expected expenditures of the entire project. That way, if you should see your trusty contractor covertly stuffing extra rolls of insulation into the back of his truck, you'll know you've got a problem.