Additional materials is always ordered to the job site to ensure enough is on hand to complete the project. When it is completed, they are returned to the distributor.
Materials can get damaged during the handling process by distributors when loading the roof, or during the installation process itself.
A small amount of additional material is always ordered for your project. If you are charged for 24 squares, may order 25 or 26 depending on the complexity of your home.
The excess is returned back to suppliers and used on the subsequent project.
It is always better to have more material than is needed to complete your project, than to not have enough and a damaged or torn shingle be used to complete the job.