Its just best practice to always make sure the pump is zero'ed before they start.
If the nozzle has been put back in it's cradle or there has been too long between filling, the systems won't allow it to add to the total, it has to start from zero.
I once saw a guy getting quite upset because he had a whole lot of containers and part of the way through the attendant took too long so the pump stopped and had to be reset to zero to pump again - he was also a bit of a tosser because he only wanted to pay 1 bill, not 2.