It is simplified: What the girl has plenty of is cartilage, which you cannot see on standard x-rays, and all bones start out as cartilage and then ossify into bone. Cartilage requires depositing of calcium into it in order to become bone; her body does not know how to deposit calcium into the cartilage, so that is what you see. No "bones" but she does have a calcium-free cartilaginous skeleton. Her head would be as soft as a ripe melon though.
Thank you. I was surprised about how many people didn't know about cartilage. Our noses and ears have shapes, despite not having bones . . .