I’ve noticed big differences (±10%) in the average power output recorded by Zwift and my Garmin Edge in Garmin Connect. I just did a 45min 26,7km ride on Zwift with an average of 199watts according to Zwift and an average of 219watts according to my Garmin Edge. The differences in normalized power is even bigger, 215 vs 248 watts, but this might have got to do with the way each platform calculates the NP. Please note that I’ve included ‘zeroes’ on my Garmin to calculate the average power. I use an ANT+ dongle on my Macbook to record the ANT+ data and I have a Garmin Edge 520 paired to my Stages power meter. Any help or insight is very much appreciated.
Zwift reads and reports power directly however the power meter sends it. It could be that you are getting ANT+ drops though. Can you open a support ticket and include your log.txt files found in your documents/zwift folder so we can investigate further?
Also are you using a USB extension cord to get the dongle as close as possible to the stages power meter? How far exactly is the dongle from the power meter?