The way servos work is that they take their position information from the internal post and compare that with the signal coming from the radio. If one is different from the other, the electronickery tells the motor to drive the pot to match up. As the two numbers get closer, the motor speed drops, mostly to prevent overshooting and hunting.
With a multi-turn winch, this is much more noticeable, and probably what you are seeing. Unless there is a great deal more load at that end of the travel. Either a load imposed by the effects of the sails or maybe, if pulling in, things getting a bit crowded between drum and shroud. Shrouded drums seem to be intended for line about 0.5mm, the more turns, the thinner the line needs to be.
