We've found that using zPower on mag trainers with a resistance level adjustment cord can give varying power readings based on the way the cord is positioned.
Symptoms of this issue can include power readings being higher or lower than normal.
There is also a known limitation with mag trainers that can cap their wattage at a point lower than the physical maximum possible with the trainer. This is due to the way zPower is calculated based on a specific resistance level and the maximum wattage that's possible at that resistance level.
We recommend leaving the shifter cable in a neutral position (flat on the ground). If your readings still feel inaccurate, ignore the recommended resistance level and adjust it to one where your readings feel more accurate.
Note that these are limitations of the trainer, not Zwift. Ideally, we'll be able to add additional support for other resistance levels on the mag trainers we support, but we don't have any dates on when or if this will happen given our limited resources as a small studio.
We appreciate your understanding. Ride On!