^^ I'm not 100% certain of this but I suspect the greater errors on the way up are due to the GPS continually finding and using different satellites as they fly through the sky. Over a small period of time the position reports shouldn't vary much but each time a new satellite is used some error will be introduced. This is why even when stationary you'll notice the position (and speed) bouncing around. On the ride down, since you are going much faster successive points are using the same set of satellites so there is less error.

Just my theory.