Pump price vs. cash register price

Yesterday I filled a gas can for my generator. I noticed the price on the pump read .809/L. The can was a 20L can so I knew it wouldn't hold $20 worth so I stopped it at $15. I went into the store and paid the $15 and didn't pay any attention to the receipt at the time. When I got home, I was telling the wife about the price being down to .809/L. Then I happened to glance at the receipt. To my amazement, the receipt said that I paid .856/L. After seeing this, my wife checked the receipt she had from a fill-up the previous day, which she said that the price showing on the pump was .82/L. Her receipt also said that she paid .856/L. These were 2 different stations and companies. Can anyone explain what is going on with this? I don't believe that I have ever checked my receipts as to price/L before. If the pump said that I had $40, I paid the $40 and went on my way thinking that I get my $40 worth. Now I'm wondering if some dealers have found a way to keep their price up without the consumer knowing it. I know I will be checking my receipts much more closely from now on.