I always thought there was a vapor catch in the pump somewhere and it is designed to catch any vapor that may have come from the giant tank underground and recirculate it back into the tank, so that you don't get this bubble of air/vapor into your tank and have the pump count it as gas. I've noticed that I do get these bubbles out the nozzle when I pump at full speeds, so I put it on the lowest setting to allow it to trap all those vapors and get me more gas.
Plus, I get more time to get those bugs off the windshield!
