It's both under- and over-diagnosed because the funnel for children is teachers who have basically been trained to refer the "problem children" for diagnosis.

I have a son who has extreme anxiety, and kept getting referred by teachers for ADHD. Hyper-vigilance for danger can make it hard to focus on arithmetic, particularly in an elementary school classroom. The psychiatrist ended up putting a note in his file because this was happening.

Meanwhile, I also have a daughter who is so obviously ADHD: she forgets to turn in her homework; there were 6 of her jackets in the lost and found in October; she will go to school without her eyeglasses; &c. But, she doesn't disrupt the classroom and is otherwise seen as a "good kid" so she was never referred by a teacher.