I understand that in case of KICKR the trainer does all the calculations and returns the speed. I ran Garmin unit in parallel and noticed that speed displayed in Zfift differs from the one reported by Garmin. It is more or less the same under static conditions on flat but during hard acceleration there is a lag and big difference for a while. I suspect this is because Zwift is doing their own calculations (deriving speed from power and other variables).

I do the same thing. I find the Zwift low numbers (< about 15mph) too low and the Zwift high numbers (> about 23mph) to high when compared to Garmin.