I remember while using bicycle with gears in childhood, when high gear was selected, the bicycle used to travel more in one pedal rotation as compared to low gear. Also more force was needed to move the bicycle in higher gear.
I guess this is the reason that auto is a bit sluggish as compared to it's manual counterpart. Because gear ratio is higher and more force is needed by engine to move it. Hence reducing fuel economy and acceleration as compared to manual.
This is a little theory which I just made, may be right or wrong......