delta3 has it pretty close. The "permanent droop" is built into the governor to make it stable and thus prevent the rpm from hunting up and down around the set rpm. If there was no permanant droop, the governor would behave like a dropping a steel ball on a steel plate, it would bounce forever. The premanent droop is like a pillow on the plate, it allows the ball to sink slightly.
A FADEC digital control has the mathmetical sophistication to use integrators as delta3 describes, so it can be truly "isochronus" - constant rpm - and still be very stable.