In an analog servo the voltage applied to the motor is anywhere between 0 and full battery voltage (either direction). It only draws a small amount of current to maintain position unless there is a heavy load on the surface. In a digital servo the the voltage applied to the motor is either 0 or full battery voltage, nothing between. It is pulsed (in microseconds) by the processor to control the correct position. It is almost constantly moving (buzzing) incrementally in minute bursts to remain in the desired position which is one of the reasons why it draws more current.