While the glass used for touch screens is meant to take a lot of abuse, it is not immune to scratching. It can happen in many ways. Allowing another person to use it, even briefly, while they are wearing a diamond ring, will be a possiblility. Leaving it set on the couch with a pet around. There are a lot of little acts that can lend to an accidental scratch.
While I am here, a couple of things. It is pretty certain that Apple is using Gorilla Glass, it is best for Apple not to advertise any component manufacturers. If there is falling out, or inability of a manufacturer, then Apple can go to another manufacturer and get the same or better product without any media attention or lawsuites. Also, Apple and others are looking at newer coatings for the glass that not only repels dirt and bodyoils, but also is self healing, eliminating minor scratches.