Fitbit not measuring distance accurately but Strava is?

[Kits19]

I started a walk today that was around 6 miles long (on OS Map). So when I set off I started my fitbit versa 2 to record walking exercise - waited for it to buzz and say its connected to my phone and started walking. When I finished my walk it said it was only 4.3 miles.

I have my fitbit linked to strava and when it was imported into strava it said it was 5.77 miles. I don't know how the map looks identical and the distance is 1.3 miles off. Even though strava gets my data from my fitbit?

Any help would be great

[Nikki02]

At least you can get it on to Strava!! Any ideas how I can make sure my runs log onto Strava? 

[Kits19]

 

 and again today. Both of this data came direct from my Fitbit (didn't do a separate strava workout). The one on the fitbit app is 0.6 miles out?!