The trends are garbage too. According to my watch my VO2Max has swung wildly between 55 and 45 over the last year, while every other measurement points to my fitness staying mostly steady.
Mine swings around as well, and I attribute it to the fact that Apple only uses Outdoor Walk, Outdoor Run, and Hiking to estimate VO2Max. Mine trends down in the winter months when most of my intense cardio is happening indoors on the treadmill or elliptical machine, and most of my outdoor activities are leisurely walks with the dog in poor conditions.
