You can't always believe what the pump tells you.
We only ever had one fuel station in out town until a supermarket started selling fuel at 10p a litre less than the old boy up the road. Funny old thing, he dropped his price to match, so wanting to support the independents, I filled up at his place. According to his pump, I got 63 litres into my Mk4 Golfs 55 litre tank. Needless to say, I've never been back.