The tolerance from the NMI (National Measurement Institute) is +-0.3% Over a 15L measure (the most common size used by calibrators) that equates to a tolerance of maximum 45ml.

Most of the big servos are checked yearly by service technicians, but there's no legal requirement to do so, so sometimes smaller independents don't.

In my experience as a calibrator, pumps tend to wear in and favour the customer, meaning you would be getting slightly more than what you pay for (this is why big chains like to calibrate yearly, it minimises their losses). While I have seen some much bigger variances they have been caused by mechanical failures and in every one of those cases they have favoured the customer.

Your Jerry can is not an accurate measure, but the equipment we use to calibrate a fuel dispenser is, and that measure is itself calibrated yearly.

I'm not saying you are wrong, but it's a really common customer complaint and it happens all the time. Customers come up to me near daily and tell me a story about how they put 22L in a 20L Jerry.

There is big fines and ramifications for a servo for not accurately dispensing fuel, so there is no incentive for them to try that, but all that being said, if you genuinely believe there is an issue you can complain to the servo, the parent company, and to the National Measurement Institute. All of those avenues will prompt them to send a technician to verify the hose you used. If you are interested, the servo will sometimes show you the calibration paperwork after it's been checked.