First off, thanks for all the clarifications guys.
I understand that many factors can alter the estimations. Still, though, I think the estimations are not 100% reliable and they will fluctuate, of course. I just think that it's weird the estimated laps number on the drivers hub changes all of a sudden.
So I'll do as Kevin mentioned, and will try to record my next races to see if the amount of litres changed or just the estimation. If the litres refueled are different, then it's definitely a bug.
Thanks a lot!