When riding your SB20 on Zwift, Zwift uses the power data from your bike (measured on your crank-based power meters) in conjunction with your entered body weight to calculate speed based on their own algorithm. 

If you are experiencing strange speed values, you may want to zero reset your power meters on the StagesBike following these instructions from our user guide. In addition to checking your power meters, ensure your body weight is set correctly in your Zwift account profile under Settings > Profile, then update your Weight.