Unfair fuel charge

I have logged a complaint regarding the fact that we will not receive our full deposit due to fuel costs - I do not understand how it is possible since we filled the tank 2kms from the airport and now first car rental is charging us for 7.1 litres of petrol - I am outraged at this. Apparently we are now paying for trips we didn't even take. <br> All I got from the query I logged was proof of them filling the tank, I watched as the tank was filled at the station, and we calculated according to the mileage used - this amount does not make any sense?