As I recently graduated from "kid" status myself, I personally would say that, at that a young age, it might well just be kids being kids. Personally, I can see how someone could sit down and breeze through a book, yet not have the concentration to do HW... its that one is something done for fun or recreation, and the other is something that the person has to do.
I guess its analogous to a mechanic who has a car hobby. When he (or she) goes to work to work on cars, its boring and they do not necesarily want to. When they come home to work on their car, its fun and entertaining, because its what they want to do.
It is human nature - how you deal with it is up to you...
