What kind of products/services do you support? That would help us to provide relevant examples.
Customer usage means the use to which the many varied customers will use the product. In the car example, Hertz would use it for fleet rentals. I would use it for mostly short duration highway driving. Another home healthcare professional would use it for many short duration city trips in stop and go traffic. The differences in this usage has a major impact on the vehicle.
Piece to piece variation includes the variation within the design tolerances and all component variation seen from the suppliers as well as all assembly variation seen in manufacturing. Again, this variation impacts the final product's performance and life.