If power data is available, as it is from the StagesBike, Zwift will use that power data and their own algorithm to determine the avatar speed in their virtual world. 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: https://manuals.stagescycling.com/en/stages-bike/user-guide/stages-link-app/zero-reset/