Originally Posted by g321

Most likely your data is including zeroes.

I have less than 10 posts so I can't post the link from the Strava support forum but Strava manipulating bike computer data is a known phenomenon dating back to 2016 (and the thread continues to the present).

Here's an excerpt from the Strava forum:

Weighted Average Power is similar to - but NOT the same as - Garmin's Normalized Power.Both numbers attempt to capture the average effort of a workout by taking into account lactate accumulation and other types of fatigue. Strava uses an algorithm very similar to xPower for computing Weighted Average Power.

There will be differences between the Weighted Average Power and Normalize Power. How much and which direction will depend on the ride. Typically, for shorter rides, Weighted Average Power will be lower than Normalized Power. This is a result of the different types of smoothing used by the algorithms. Both numbers have been used successfully by professional coaches.

As to the question of why Strava can't simply copy values exactly from the Garmin file in regard to average power and normalized power: Well, our engineers have worked hard to process the raw data in the file, clean it up, and use algorithms that are standard across any file created by any compatible Strava device. It's just the way we have always approached activity data. We don't have plans to change or disable these core functions and algorithms in our data processing.