years.
Different technologies. A conventional automatic transmission has a torque converter so the car does not stall at low speed or idling, and herein lies the big weakness. Torque converters are a fluid couplings, the TC typically converts some of the motion to heat, which is an energy loss, and reduces the efficiency of the automatic transmission somewhat (although now days, with lockup torque converters, the difference is smaller). There are also other losses in the automatic transmission besides the torque converter, but the TC is the biggest power-efficiency loss.
An automated manual transmission has one or two mechanical clutches and functionally works similar to a conventional manual transmission, but the clutch and gears can be electronicly controlled. There is no clutch pedal, though some systems allow manual gear selection (usually sequentially like a motorcycle). Most also have a way to keep the car from stalling when the engine idles, and to simulate the "creep" that a torque converter produces. These transmissions should theoretically be more efficient than a manual transmission, especially when shift points are selected by the car's computer rather than by the driver, and the transmissions also shift faster from gear to gear than any human ever could, resulting in better acceleration.
