On several watches like the Vantage V, Grit X and PacerPro, Polar offers power to runners based on GPS and wrist motion. The Vantage series watches are also compatible with Stryd. So, Vantage V users have two options. Due to different considerations, the power number of Stryd versus Polar may be different. Typically, the Vantage V will report a power number that is 25% higher as they do not consider the rebound effect that allows runners to recycle energy.
You can read more about in this article from Hans van Dijk and Ron van Megen.