Ever notice how the folks who say "you don't understand" when you disagree with them, don't understand how anyone could disagree with them?
Translation of "you don't understand":
1) You lack the ability to understand. i.e. U R dumm
2) I'm so much smarter than you that it's not worth my time to explain it. i.e. I M smard.
3) I don't really understand so I can't explain it to you.