I spent 30 years as Chief Engineer at Sea. Japan, North America and UK were pretty good, Japan you often got a little more than you thought you should.
Most owners/ operators get their fuel through a broker who searches the World for the cheapest price per ton. How can you sell your fuel cheaper than anyone else, who buys the fuel from the same refinery? Simples, ruthlessly cut costs and give short measure.
In Singapore we had a Bunker surveyor, but the Bunker companies would only permit the surveyors they approved to carry out a survey on their barges. Guess whose side the surveyor was on?
When I have refused to sign the delivery noter I have been told that if I want more fuel they will just top up our tanks with Sea Water. I don't know if there is an answer, I never found one.
Some companies have their own meters installed in the Bunkering pipeline, standard on supply boats so that you can measure how much you are selling to the rigs.
However the common ways of fooling that are opening an air cock on the fuel transfer pump so that air is drawn through the pump and thoroughly mixed with the fuel, increasing its volume as it goes through the meter and when you take ullages after bunkering, but after a couple of days, the air separates out and all the tank levels drop. If you have a system of measuring the tank levels remotely, based on the pressure at the bottom of the tank there will be a discrepancy because the addition of air obviously reduces the SG of the fuel delivered. So while the remote tank level reading will stay the same the ullages will increase.
It is always better to ask a stupid question than to do a stupid thing.