The treadmill is accurate. Fitbit uses your stride length to compute the distance and stride very rarely is exactly the same for every step. There is no way to know for tracker what distance you ran on the treadmill.
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I would say: it doesn’t really matter, because distance isn’t such an important metrics (unless you’re training for running events, for instance). I would pay more attention to things like calories burned, active minutes, cardio fitness score etc., and how they evolve over time.
Dominique | Finland
Ionic, Aria, Flyer, TrendWeight | Windows 7, OS X 10.13.5 | Motorola Moto G6 (Android 9), iPad Air (iOS 12.4.4)
Take a look at the Fitbit help site for further assistance and information.