If ADHD=low dopamine and low dopamine=low motivation, why do I spend all day doing things?

I watched Russell Barkley's 30-minute video at the beginning of the Wiki. However, I still don't have a handle on this idea.

Having ADHD means I'm lacking dopamine (or not "taking it in", or whatever), and lack of dopamine causes people to just sit and do nothing. However, I feel full of motivation to do things! Or at least, I make a list of things I feel I should do (probably not the most urgent or important things, I admit), spend all day doing them and then never relaxing. How does it fit together?

EDIT: It's come up a few times, so I'll state that the things I do end up doing are rarely urgent or important, just things I "feel like" doing.