Question:  Why do most ADHD individuals have symptoms of different subtypes?  

Answer:  Models have been developed to show how a person can inherit one subtype of ADHD and develop symptoms of the other over time.  This helps us to understand the fact that the majority of ADHD patients have some symptoms of both.  Problems with focus, attention, and concentration can be associated with both types.  In one the brain won't turn on - “not interested”.  In the other there's too much noise and the individual can't focus on the important because of too many unimportant distractions.