Do the pennies add up?

For the second time in a month I have filled my car with petrol, noted the cost and then found that the price charged was a penny more than was on the pump. Now who complains over a penny on a £50+ petrol bill, not me? Even more oddly, when I get back to the pump the total now reads the penny higher figure.

So what's happening? Is this a scam or are petrol pumps now adding fuel after the nozzle has been holstered? I know it's only a penny per transaction but if it's on every transaction every day...