Unexpected unfair refueling costs on return of car

I returned the rental vehicle with a full tank of fuel as required by the contract. I refueled up to the first click (as per the AA recommendation ). I was then charged for a further 12 litres of petrol. The explanation I received, was that they filled the tank to the brim, and that there was an airlock which allowed them to add additional fuel. We drove 503kms, the vehicles fuel consumption as per the dealers website indicate that its a 45 litre tank, with a fuel consumption of 5.1L/100kms. According to these specs we used approximately 25 litres. When we refilled the tank before returning the car it reached full capacity at 19 liters, which means that if they provided us a car that had a full tank the maximum amount of fuel that they could put in would be 6L. Upon challenging this, First Car Rental stated that we had to fill the car as per their contract, and considers the case closed without explaination of the discrepancy. We feel that we are bieng ********** as the 12L (more than 1/4 of the tank capacity) is excessive.