When JOIN imports power data from Strava, it does not use the average power or the weighted power. Instead, it uses a calculation called "Normalized power." Normalized power is actually an algorithm that calculates the physiological load of intervals compared to a steady power output. It was introduced by TrainingPeaks and is used to express the difference in load on the body of a steady power output and a ride with a lot of variation in power. The physiological strain and differences in intensity of a steady power output or a lot of variation in training sessions are not well expressed when you only look at the average power of these rides. In this case normalized power would come in handy. This results in a more accurate representation of the average power output during the entire ride.
More info here on Normalized power - https://join.cc/cycling-tips/why-do-you-use-np-for-your-training/