Difference in data between Zwift and Garmin 935 for same sessions

Riding indoors on a Tacx Flux with Zwift and also using a garmin 935. On every occasion the distance and power recorded by Zwift is higher than the 935. I recently bought an Assioma pedal to check outdoor and the Zwift data seems to be more reliable. It must be something to do with how Garmin is interpreting the data from Zwift but why the big difference?