Well, the major issue resulting in over/misdiagnosis with ADHD is the tendency to ignore the stipulation that those diagnosed suffer from symptoms that "interfere with, or reduce the quality of, social, academic, or occupational functioning," and that "several inattentive or hyperactive-impulsive symptoms are present in two or more settings." With "several" in this case meaning 6 to 9 symptoms.
It's an exceptionally high bar.
As a (hugely reductive) rule of thumb, if the child in question can focus on more than one non-passive (ie. TV) preferred activity for a sustained amount of time... they generally don't meet the diagnostic criteria for ADHD.
Which is not to say that they won't be misdiagnosed...
