It's been close enough to my Garmin for me to stop worrying about calorie expenditure.
Have you been tested at a sports lab for a more accurate estimate?
I have deconstructed the AW data to see that it has errors between BMR and active calories. Furthermore, it is off from every other calculation device I have looked at, including Fitbit, Garmin Edge 520 with and without power, Garmin 910XT (running and cycling with and without power), Kurt Kinetic InRide with power, and multiple treadmills. The Garmins align with the other devices. Garmin licenses the FirstBeat algorithms for calorie estimation, and FirstBeat is highly regarded.
Furthermore, there are a number of other posts from athletes on this forum who have found that the AW does not match their devices, treadmills, and stair climbers.
Furthermore, AW calorie calculations are off from numerous online calorie calculators and published, peer-review studies that have analyzed actual calorie burn in a controlled method.
I think the AW calorie calorie calculation is like a broken watch: if it happens to match another calorie counter, it is probably random luck.
This was just my little rant that I am frustrated with Apple for doing such a crappy job with the fitness and activity aspects of the AW.