It's definitely overdiagnosed. My wife's niece was properly diagnosed with ADHD. She is mentally retarded but with an aggressive bent. Her father, who was not the husband at the time, died in prison and the mom was also difficult, so there is a family history of problems.
The part about little boys having more ADHD is bogus. My daughter is almost five so I get to see the behavior of kids all the time. Yes, most of the boys misbehave but it is due to two things: the moms have no concept of discipline and the kids don't have sleep routines.
The lack of meaningful discipline causes most of the problems. I don't know how many times I hear moms say "Stop doing that, stop doing that ... 1 ... 2 ... 3", then nothing happens. The kids realize that there are no consequences to bad behavior. They have no guidance on good behavior. Kids naturally want to stretch boundaries and its our job as parents to show them the boundaries. You tell your child that what they're doing is wrong *one time* and what the consequences will be if the behavior continues. If they continue to misbehave, then you begin the consequences *without* negotiation.
The other cause of bad behavior is the lack of regular sleeping schedules. Kids need a routine so that they get a good nights sleep. When our daughter's schedule gets disrupted, she displays some of the ADHD symptoms the next day (defiance, overly emotional). Many if not most of the families around here have chaotic sleep schedules. Their kids don't go to sleep at a relatively fixed time, etc. so they are just tired all day.
Anyway, I'll get off my soapbox now...
Believe me, we know this. We are very disciplined with our parenting approach. Our kids are fine on the discipline front, we are consistent and firm. And I agree that ADD is overdiagnosed. Without a doubt it is something that helps drug companies push unnecessary medicine and lazy parents a way to control challenging children. But to those who really do have it, and the people around them, there is positively no guessing involved. Maybe in the past they had other names for it, but ADD is very real to those who really have to deal with it.
Perhaps not what you are talking about, but certain types of misbehaving are simply "tactile learning" in an environment set up for girls (who sometimes have the same tendency, but at a much lower rate).
Inability to pay attention at all would be rare and for cases like this girl you describe -- a true learning disability. Much of what is wrongly called ADD is really just inability to pay attention to a particular teacher or the ability to pay attention to several things at the same time --- which looks like not paying attention. A child can be sitting at his desk, paying attention to the eraser on his pencil, paying attention to the kid sitting behind him, and to what is going on outside the window, and to his plans for after school, and to the fly on the wall, and not paying too much attention to the teacher. This child is bored but doesn't really have a learning disability --- sometimes this type might also be paying attention to the teacher and learning just fine but the teacher doesn't think he's paying attention because he's a multi-tasker --- capable of paying attention to several things at one time.