This kind of juvenile injury is known as «Toddler Fracture» in humans, and it's thought to be caused by the fact that the outside, fibrous layer of the bone (periosteum) is relatively strong in relation to the elastic bone inside. (americanshihtzuclub.org)
In addition, the abscess has a thickened, fibrous layer encapsulating the pus. (petsmagazine.com.sg)
The fibrous layer over the baby's head is strong enough to withstand normal hair washing, brushing and touching. (howtoadult.com)