Feeling puzzled!

For a while I have been very puzzled as to why my daughter always seems to record more steps on her Fitbit on those days when either we’ve done the same amount of exercise or I have done more! We have exactly the same Fitbit, the only difference is mine is a year older! 

So today I decided to carry out a scientific investigation with my younger daughter. She put a watch on each wrist and went about her daily business, which mainly involved walking about a bit, chatting and eating. 

Of course, we recorded the starting points for both watches. My elder daughter has not synced her Fitbit recently, hence the incorrect time. Her watch started on 317 steps, mine on 928 steps.

When we checked the steps after a short period of time, my daughter’s watch was at 646 and mine was at 1,134 (bit blurry).

This meant that the black watch (my daughter’s) recorded 329 steps and mine, the purple one, recorded 206 steps. That’s a difference of 123 steps! And if you calculate that over the course of a day, it probably works out as a difference of a few thousand steps if we’re covering the same distance!

So my question is, why is the exact same make of watch recording steps differently? Is it because one watch is older than the other? And if so, surely that shouldn’t make a difference, as it would mean that after a short period of time, the Fitbit becomes a lot less effective!

Any comments are welcome!