Can't increase time on Fitbit Versa Lite and Strava says I am doing circa 10% longer in same time?

[TJWKH1994]

Hi All, 

I am training to run a marathon on behalf of my mum, who sadly passed very prematurely and quickly - her last gift to me was a Fitbit Versa Lite - so it holds good sentimental value. I was meant to be running London Marathon 2020 in 2 weeks time, but cancelled due to Covid. I was not even a runner before this - so I was not caring about what time I was doing whatsoever when training initially, and purely focusing on getting miles into my legs. Now it's put back, I am focusing more on my timings and finding it almost impossible to get fast than 5'40 km, and almost all my runs I range from 5'40 - 6'00 km. I started doing some short runs at high tiempo and noticed that the Fitbit was still saying my first KM was almost always 5'50 - 6'00 KM despite really going for it. I then got Starva, and ran today and yesterday, here is the results:

Yesterday:

- Strava: 3.17 miles/ 5.1km = 26 mins 44 seconds 

- Fitbit: 4.6km = 26 mins 45 seconds

Today: 

- Strava: 5.99 miles/ 9.63km = 52 mins 49 seconds

- Fitbit: 8.9 km = 52 mins 47 seconds 

Does anyone have any idea why this is? And which one is likely to be right? 

[Rich_Laue]

I would use Google pedometer to check the distance.