This is common with every GPS watch that also has an internal accelerometer. Without being able to create a baseline of your running style using GPS to track actual mileage vs the movement of your wrist, the watch is guessing. Probably guessing based on height/weight based averages. So it'd be close for some people and not so close for others. Do a few outdoor runs with GPS and it should tune in to you a lot better.It should learn more and more about your running style if you run with it outside using GPS before using it inside at treadmill.