Steps

[Instrumental1]

The Fitbit 5 and I would presume ALL there others count STEPS when your ARM/Wrist is moved, 

NOT actual Steps

SO, One’s ACTUAL  Steps are really arm movements, Ie; Writing an email will ‘UP’ your Steps, Waving, Cooking, ANYTHING whereby you use your hand with FB on will add steps.

I wrote too  FB about this No reply

The  Fitbit should l/will be more accurate wearing it on your Ankle.

[JohnnyRow]

This is well known of any tracker worn on wrist.  Fitbit does have a few models (Inspire 2 and 3) that can be worn on a clip as on a belt, probably for better step accuracy, but then losing all heart rate measurements and ability to see data on your wrist, if that is your preference.

There's no way anything on your wrist can really know precisely what your legs are doing.  Fitbit has filters to try to take out some non-step movements.  For most people over the course of a day,  total step count pretty much evens out.  Exception might be for obvious steady repetitive arm movements such as strumming a guitar, knitting, playing drums.  When you speak of being more accurate worn on ankle, it sounds like your only concern is step count, and you are thinking of your Fitbit as a pedometer.  If that is really your only concern, you might even put it in your pocket, detecting hip movements.  Some who are not allowed anything on arms do try wearing on ankle, probably with a strap extender, preferable under a sock so won't get lost if falls off, but likely won't get good hear rate then if worn on ankle for extended time.  If you think of your Fitbit as a pedometer, that might be fine for you, but then you are missing out on many of its benefits in order to hopefully get a more accurate step count.