Getting the right amount of FUEL?

I am still puzzled (see Dec story) by the short-delivery of fuel from the petrol stations where I get my mogas (95 unleaded car petrol from the same stations you fill up your car) for my aeroplane. Not getting any consistent quantity of fuel in my container, I brought it inside to check my measurements with a measuring jug. This proved that my original lines on the petrol container were more or less correct and that I am being short-changed when I refill the container.

The exception was Saturday when my container was again fiulled up to the line... it almost seems that the measuring device the petrol stations use depends on the weather or air pressure, so that when the QNH is lower you get more fuel? Is this possible? How do the petrol station's bowser dispensers work and does air pressure affect the accuracy of these?

I have asked Wendy Knowler of East Coast Radio's Consumer Watch to investigate, as at R11 a litre, a 2l shortage on every 25l fuel sold means about R25 per car tankful of petrol consumers are being overcharged nationwide (when the weather is good?) Does the sunny-day filling up of your tank subsidise those who fill up on a rainy day?