There is no calculation. Most stations have level sensors in the underground tanks. It was my job to "stick" the tanks. Every night I had to open the tanks and measure how many inches of gasoline were in the tank. You then cross referenced it with a chart calibrated for that tank. No calculations to be done. If you had 36" of fuel in the tank you read down the chart to 36" and then wrote down the corresponding number of gallons. I then compared this with what the tank monitoring system reported. I also did the nightly reports where I compared the day's sales compared to the tank level so we could detect tank leaks, or water in the tanks.
A 500 gallon difference in volume - I have trouble with that.
Difference in gallons pumped due to temp - I have trouble with that.
Putting the exact same amount of gas in your tank at fill-ups at different stations - impossible.
What I think is that since all tanks/pumping systems are calibrated/regulated, if there is a discrepancy someone at the gas station is ajusting something in their favor. That is cheating the customer. I am sure it is done but I have to believe most places are pumping what the pumps say.
But pumping the gas slower makes sense to me. That is interesting. The gas is displacing air in your tank so if your particular vent cannot let the air out fast enough because you are pumping at full blast/pressure, then the pump could click off faster and then you could usually force feed another gallon in there.
For the most part stations keep their pumps calibrated. Intentionally rigging the pumps to deliver less fuel than indicated will cause a discrepancy in the amount of fuel that is in the station's tank, and how much should be in there. If your monthly over/short gallons is too much the EPA will start crawling all over the place looking for leaks into/out of your tanks. Even if nothing is wrong having EPA come in on a weekly or monthly basis is unnerving and should be avoided at all costs. Its best to operate your station properly and not give those EPA pencil pushers any reason to make an appearance in person.
I use my Dashdyno to predict how much gas I should fill up and it can tell me if pumps are shortchanging me. I've run into many bad pumps and got ripped off several times. But the worst was a Phillips gas station that advertised the lowest price around, but when I compared my receipt to my credit car statements they charged me higher than the receipt!
Some stations here charge for using a credit/debit card, that might be what the extra charge would be. Some give a discount for cash. After this tank is over i will just put 5 gallons each time so i can get a true reading. As the 5 gallons runs out i will fill my 5 gallon container so when i run out i can put in the 5 gallons. Will try this maybe six or seven times and see what happens.