Not Updating Your Health Data
The Apple Watch uses a variety of metrics and sensors to determine how much you move and how many calories you burn doing it. But the Apple Watch’s fitness and movement accuracy is largely dependent on your health profile, as well as several other factors.
You should definitely keep your Health Profile data up-to-date inside of the Watch app, including updating your weight periodically.
You may also want to calibrate your watch by walking or running outside on a flat route for at least 20 minutes. This will help calibrate its GPS data for more accurate workouts.