Many companies sell product and share profits via contract (oil company is one example) that require verification/calibration on an agreed frequency. When money is involved all parties have a right to want accuracy to some agreed standard. The gas pump you use most likely has a dated calibration sticker on it showing it last being checked/measured. The equipment used to check such gas pumps also has to have it's own calibration/verification process. It can be a very big deal in industry, but for hobbyist use, it's mostly up to the individual how he/she goes about 'trusting' their measurement equipment.
I knew one garage where the one pump had some internal wear, in that it drooped quite badly at a low flow rate from fuel bypassing the positive displacement pump seals internally. Give it a good flow rate, like you normally used to fill the tank, and it was accurate. Slow down to a trickle, like I did when the tank breather blocked with dirt, and I filled the tank from empty to full in 10 minutes, but only put in, according to the fuel display, 10l. I did phone the regional office the next day, talked to technical services and told them of the issue ( here in South Africa the fuel stations are owned by an oil company, and the pumps are oil company property, not part of the station, which is a separate business) and about a wek later when I went in there that pump was a complete new one, a complete replacement from the floor up of the new styling as opposed to the rest which were 2 stylings old.
All Tokheim pumps in there, I have seen some which, in a brand new skin, were older than me, having manufacturing dates from the 1950's, and still having a mechanical gallon counter on the pump itself, which has been around the clock a lot of times. Old pump, new shiny outside wreapped around it.