All the answers so far probably have validity, however, here in the U.S. an alternate answer is plausible... Back in the day, when the gasoline was sold from the distributor to the vendor, the gas was placed in above ground tanks with ill fitting lids on them at the vendors store (often a hardware or other coutnry store). The pumps were a hand driven affair that pumped the gas into a tall glass containe on top of the hand driven pump. The glass container was marked in graduations for gallons, up to about 10 gallons max. All this greatly contributed to the shrinkage of the volume delivered from the initial supplier through evaporation. To make up for this loss, someone came up with a typical, average, summer time evaporation rate loss. The mathmatics involved amounted to 9/10ths of a cent per gallon. Hence, the final seller sold his product at so many cents per gallon "plus" 9/10ths of a cent (US) per gallon tacked on for the shrinkage...
The added near penny was retained when mechanical pumps were introduced and underground storage was invented. (By the way, we all know the best time of the day to buy petrol (as ya'll call it) is early in the morning or late at night when it's cool, don't we?)