Nope, not scratch. Scars.

In the first chapter Michael sees his first group of creatures and says they were covered in Scars and Bruises. As a vet of three tours he is going to know the difference between a new wound and a scar. If scar is really what KC ment, then these are "older" zombies that have been around. Maybe even in cold storage and released. Alternatively, these zombies have super healing powers and wounds are not ignored and left open for cinematic effect. Then again, maybe scars was not quite the right word.