It's all marketing BS. The Volt's "MPG" is entirely related to the length and speed of the trip. (not unlike other hybrids that can travel a certain distance on battery power only... like the Fusion Hybrid) The term "MPG" becomes increasingly less relevant when the car can be plugged in to obtain energy from an alternate source.

The Leaf is a completely electric vehicle. I don't know how or why they even bother putting an "MPG" number on it. It burns NO fuel. Ever. Doesn't even have an engine. It's like assigning an MPG value to your refrigerator, it makes no sense.