Ex-fueler here, I don't know if this is industry wide, but what I'm describing holds true for all stations for a large company that specializes in 121 carrier fuel management.

Once you defuel an airplane, all the fuel in that tanker must then go into the company's aircraft that you de-fueled from. Say you have a truck with several thousand gallons or so in it and you would do a defuel from a 9E airplane, you would need that truck to only work 9E flights until it is empty. That can get to be a massive pain at small stations/places that don't see many flights from the affected carrier. At my ex-employer, if the fuel vendor personnel really want to follow the rules, the fueler should call their tank farm people to do a density test on the fuel in the truck immediately before de-fueling into it. Plus, I always found it funny how many of my coworkers had difficulty with the paperwork/basic math. The truck meter only counts upward, whether the fuel is coming in or out, so your gallons 'through' the meter no longer equalled the amount 'out' of the tank.

Apologies if I bored you all to death, but just figured I'd explain the rocket surgery behind a simple process.