Hai buddy its nothing wrong in it.
It just helps you to refill fuel at the right time. There's no problem in it. My fiesta does the same.
It shows the expected KMS as per the last litres mileage.
Once this happened to me
when i refill diesel for our outing in the first time it showed 720 kms
after some time it showed 630 kms.i was shocked
after driving it in the 4 tracks it became balanced but it didnt increase from 630.
but just decreased 1 km for 3 kms
so it will be balanced.