Garmin 810 & 910XT: why different powers?

I raced with my Garmin 810 and 910XT this past weekend at the Chicago Tri. Analyzing my data afterwards showed an average power difference of 29 (284 vs 315) for the two devices. Both are set to include zeros. Is there some other setting I am missing that I need to check on?

I don’t remember if I calibrated them the morning of the race, would that make much of a difference?

Calibration could account for that but check that they are both set for 1s recording and not smart recording.