I can only answer from my experience with my oil deliveries.
As regards the meter, they put in the docket, press the reset lever, which resets the count to zero and then start pumping. At the end of the fill, the docket is taken out with the stert (zero) and end fill values on it. The difference is the fill. This is the delivery docket and it is the standard way he records the delivery (and payment).
Also, he once put the hose into one of the openings on the top of the truck and pumped in a number of liters. I asked him why and he explained that the previous fill had been diesel, and he needed to clear the hose of that before he started my kerosene fill. This leads me to conclude that
1. He knows how much is contained in the hose and
2. he clears out the hose and pre-fills it as necessary. He had selected kerosene before clearing the hose so the incoming kerosene in the hose pumped out the diesel - the hose was not empty at any point.
3. You pay for what you get, and you get what you pay for.