Power measure on IOS device

I’ve just starting using ZWIFT on my iPad, and have used it for a long time on my laptop before.   On my iPad (compared to my Garmin 1000), I noticed that it captures a lower wattage by 20-30 watts generally, and also that impacts my w/kg.   Is there any calibration I need to do to use on my iPad?  It was consistently off for the entire ride.   

Any insight is appreciated.  

I don’t see why there would be any difference, both devices just pickup what’s being broadcasted. It could be the difference in smoothing, ie instant, 3 second and 10 second…