Even the pumps in the same petrol station can vary. Try filling the same 5L can, some pumps somehow manage to squeeze more than six in!
And before anyone starts saying "That can't happen!", of course it can. Little tweaks when nobody's watching. And it's no wonder when our fuel is so heavilly taxed in this country. The stations only earn about a penny a litre. It's disgusting how we're taxed in this country. But you'll all vote for lower income tax next election, I bet, and then be robbed in the price of your goods instead.
Go ahead, calculate your fuel to the n'th degree. That's not where the wallet heammorage is occuring.