Why do some drivers in the carpool lane driver slower than the traffic in the regular lanes?

Sometimes its because they were there to get out of a traffic jam, and the jam has ended, and normal freeway speed has picked up. In that instance, the driver should pull out onto the normal freeway and get over to the #3 lane, as soon as safely possible.

But other than that circumstance, why would anyone enter a carpool lane, than proceed to drive slower than the traffic in the #1 regular lane?

Are they being stupid? Or egostically evil?