Because these devices do not actually count steps, but the movement of the arm on which you are wearing them. And then assume the movement is related to you moving around as in walking. There is no technology available yet to address this issue. They are pretty accurate in counting actual steps when you walk or run though. If you track a walk or a run, then the step count for that workout would be pretty accurate. So, if you only wore the device while actually moving around, and take it off when you are not moving, the step count would be pretty close to the steps you actually took. But if you wear it all the time, as it makes sense for other reasons, the step count will be unavoidably higher than the actual steps taken. It is therefore also advised to wear these devices on the non-dominant arm, because we usually use and move that arm less often.
