Why is a manual transmission more fuel efficient?

This used to be the case but not always now. Originally all automatics used a fluid flywheel.Oil was forced from one circular vane connected to the engine(similar to the turbine on a jet engine) to another on the gearbox.As it was a fluid that drove the transmission, some loss occured in the transfer from the propellor(on the engine)to the impellor(connected to the transmission.)So more power was needed from the engine (so more fuel) to compensate for this loss. Some modern autos use a mechanical clutch which transfers most of the power - the VW Golf auto for example - so that very little power is lost in the process.So fuel consumption is very similiar.