I found out, that the distance readings from Zwift are different to the readings my Garmin Edge 1000 logs. The Edge shows round about 18-20% less distance that Zwift. How does Zwift read the distnace compared to a garmin device… Wattage is almost the same! (this happens on a Wahoo Kickr and also on a Tacx Neo!)
Distance on a turbo is an irrelevant measure. Zwift uses watts only and converts that into speed (and hence distance) in the game. It would be surprising if distance was the same.
Yes, because the Garmin Edge 1000 does not take in to account any virtual factors that are present in Zwift. The Garmin doesn’t know you’re on Watopia or Richmond - it thinks you’re riding in a straight line on a featureless, flat landscape 
On a turbo trainer this is understandable. On a smart trainer I would rather see actual speed and distance.