Fuel **** at First Car Rental

I have rented a car on King Sharka. So then day I gave the car back, I stopped at Sasol on the airport to fuel up the car. When I returned the car they confirmed that the car is full and I signed that everything is in order. They confirmed with me that my whole deposit will be returned to me. I drove 530km and filled the car with just over 33 liters. That is 16km/l and a bit back for that 1.2 turbo at the coast and thought that maybe the car wasn’t full when I got it. Then 2 days later I got a fuel bill and they put in another 10.5 liters. So that means the car got 12.1km/l. That is the same as my Hilux in Cape town. Then they said there was a 10.5liter air pocket. So I phoned Renault in Somerset West and they said they have never heard of such a problem ever. Someone is trying so **** me. Then I filled the car it was just below half, it is a 42 liter tank. But in total it was 43.5 liters. Never rent a car from them ever. Here is my contract number - ***