Yea every fuel pump is different, most overestimate how much you put in, some are slightly under. You can test this by filling a small Jerry can that you know the exact size of. 5 litres is pretty common, fill it up and I bet you a small sum of money the pump will read higher that what you've allegedly put in. Now empty it and try it again at a different station, it will be different again. Annoying but true...