I would reckon it's due to how parents raised their deaf child(ren), whether they were exposed early to reading books and having had their deafness diagnosed early. It really depends on the circumstances the child(ren) are reared in. It varies from family to family, really.
I'm a Deaf child of hearing parents and I grew up watching my mother read book after book, plus she learned to use sign language from when I was about 6 months old, I would say. I also mainstreamed in a deaf program at a public school from grade 1 to 4 before I transferred to a Deaf state school where I completed my education.
