1) Dyslexia
2) Skill Giftedness (causing extreme boredom)
3) Sleep Apnea
You can get some links to more information on my profile page.
A vast majority of schools just lump all this into "ADD/ADHD" without bothering to learn what the true issue is the cause.
While I'm not part of the ADD-doesn't-exist group, I certainly agree with you here. It's simply easier to drug children and keep them quiet.