Changing gears in a manual is mechanical, using energy from the driver via the clutch and gearstick. In an automatic, the only place that energy can come from is fuel or battery (which is charged by fuel anyway), not to mention the computing power required to stop the automatic transmission from intrusively shifting gears and also to maximise the fuel economy. That's why autos are inherently less efficient on gas. If a driver's driving a manual such that fuel economy is less than it would be in an auto, then they aren't doing a very good job at driving.