Stryd can currently not detect the surface you are running on and adjust power readings accordingly. For example, if you run on the beach it will obviously take more effort to run at the same pace then it would be on the road. For the same effort, Stryd will indicate a lower power level on the beach.
If you always run on a particular surface, you can adjust your power zones. We recommend doing a critical power test on the surface you regularly run on. If you do that, your power zones will adjust so that you can still use power for proper training.