Power = work/time I am going to guess both Garmins treat the numerator the same and that the difference arises in the denominator, i.e., different moving times. That should be something you can check -- did the two record different moving times when they reported different power outputs? The last ride serves as a positive control -- I am assuming from your description that you didn't stop midway, or did something different in that last ride with respect to moving times.