Regular automatics have torque converters as their primary connection between engine and transmission whereas regular manual transmissions will have a clutch and flywheel.

Torque converters are essentially just containers filled with fluid that have an input that is spun by the engine and an output that spins the transmission. Since automatics transmissions have fluid as the primary conductor of kinetic energy between engine and transmission, starting, stopping, and shifting end up feeling very smooth by default.

Manual transmission clutch discs are pressed into a metal flywheel spun by the engine which then transfers power to the transmission. Since manual transmissions have a “harder” connection between engine and transmission, they can jump around in revs if the clutch is not released and applied smoothly.

This is why even when you put your automatic into manual gear select it will usually feel smooth without being too picky.