There are three functions for the fuel useage
Instant - shows what fuel you are using at that second.
current trip - shows the average fuel consumption you are getting on your current journey
Overall - shows what average fuel consumption you have been getting overall - since your last reset.
I'm not sure if you know about these, but they are easy to get to just by switching on your stalk control. If it is something else other than this it is a case of a journey to the dealer.