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It's because your charge 3 is strapped to your wrist not your foot. It works by detecting movement of the fitbit i.e. your wrist, and then tries to work out whether that movement was the result of a step or just random arm movement. It's not a precise science and it sometimes gives too many steps, other times too few. Have a look at the following help file where Fitbit explain this in more detail and describe what can be done to minimise it: Help article: How accurate are Fitbit trackers?
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