Having the same gene doesn't necessarily mean having the same outcome. Different people have alot of different factors that determine how their body reacts to something. Maybe one child has a weaker immune system, which might either be from another, completely separate gene or from exposure to disease or medication. Diet plays a big part. Two children 2 years apart might easily have completely different metabolisms based on the change in parents' opinion regarding what food is good for you.

The severity might change, then again it might not. Jonathan was alot sicker when he was younger than Ginny, but they would have the same gene for CAH. Outside factors have influenced the different results.