How do the HOV lanes work in Atlanta?

For the past year and a half, I've been commuting every day on I-85 and I-75 in the North Atlanta area, mainly ITP. I've noticed that every day during rush hour, the overwhelming majority of cars using the HOV lane only have one person in the car. I usually drive in the far left lane next to the HOV lane, so while I’m stuck in bumper to bumper traffic I pay attention to all the cars going by in the HOV lane and I've come to notice that usually about 1 out of every 15-20 cars actually has more than one person in it.

I was always under the impression that regarding cars, the HOV lane was only for drivers with more than one occupant or alternative fuel vehicles, so I’m wondering if there really are that many assholes every day who consider their time to be more important than everyone else's and the rules of the road don't apply to them, or if there are more ways to qualify for driving in the HOV lane that I'm not aware of?

EDIT: I do mention the AFV exception in my second paragraph, but it's (unfortunately) sounding like there are a lot more people in Atlanta who are incapable of self-governing than I'd have hoped.