I think a lot of our kids are just like you describe. My son who is 12 just spent a weekend at a wildlife ecology conference aimed at an adult audience at a lodge by a lake. He attended every session, took notes, asked questions and behaved like an adult. But at breaks he would go out to the lake and play and next thing you know he'd fallen in and he was soaked! He was late for meals because he was building a sand castle, he kept tracking sand into the lodge, and kept leaving things he needed in the room, in the car, or in the lodge. Some of his brain is like 20, some of it is 12, and some days I would swear he is more like 3. You have to take care of each part of their brains--the 20, the 12, and the 3 all need to be fed, nurtured,allowed to grow!
Try reading about asynchronous development and you will see that what you describe is quite common.
