I guess you haven't spent enough time around an ADHD child to know!
Well in all fairness he did say 90% of the time. And I'm inclined to agree that children are often misdiagnosed with ADHD. I have personally met people who were told, and believed that they had, ADHD. Mostly because they were very outgoing, active, ancy, talkative and somewhat rebellious. Characteristics that really have nothing to do with ADHD. They really didn't even know what ADHD was and none of them had ever undergone the more definitive diagnostic exams. Like I said earlier, I think that changing the name of the syndrome from ADD to ADHD was a bad idea because it reinforces the misconception that ADHD=hyperactivity.
I've only ever met one person who I thought had the real pathological deal. But that's irrelevant, I've never met anybody who had OCD, autism or asperger's either. Know what that means? Nothing.
Have you noticed that most people who dismiss ADD, or ADHD if you like, don't even know what it is? They'll express their opinion that it's just simply lazy parents, self discipline etc. And I'll realize that they just think it's an attitude problem or a misadjusted personality that will just go away with the right guidance. They don't know that it's a biological malady.
<edit> BTW 100 years ago adhd was called "minimal brain damage syndrome". A phrase that might turn out to be the most accurate name that the syndrome ever had.