Getting the right amount of heating oil delivered into your oil tank is a matter of trust. We trust that the oil supplier is honest and operating a correctly calibrated gauge on their delivery lorries. All it takes is for the gauge to over-read, and the unwitting customer will be delivered less oil than they are told.
This sort of thing does happen, although it is probably a rarity as the supplier needs to somehow cover up the discrepancy in wholesale oil purchased vs oil supplied to customers. One father and son company that profited from it for several years, D White & Sons, typically under-delivered by around 10%. When a Northern Ireland residential home became suspicious that it had received enough oil, the police were brought in and they confirmed that the next delivery of oil from the company was short. Police searches at the firm's office revealed that invoice and delivery data didn't tally, and the father and son were subsequently found guilty of fraud by false representation - and handed down prison sentences.
A rough way to check how much oil has been delivered is with the oil tank calculator whereby you measure the tank size and read the oil level indicator to work out the oil volume. Do this before and after your oil delivery, and you should have a good idea of the amount that's been put into your tank.