Stanley,
Your analysis appears to be missing the main point of a hybrid. Much of the energy generated by the gas that would otherwise be LOST is now recovered via the hybrid battery even if not 100% efficient. This energy would be lost entirely in the pure gas vehicle, and is now recovered in part. This is where the increased mpg comes from. The more of that otherwise lost energy you use via running on the battery, the better because less new gas has to be used.
Yes, of course all the energy is coming from the gas, but less of it is wasted because of reclamation via the battery.
Joe