The reason it’s there, that 9/10 of a cent or pence or w.e is the same reason why lot’s of prices are $XX.99. It seems like it’s a lot cheaper than the 1 cent or 1/10th of a cent that it actually is. For fuel specifically, it started to be added so that gas stations could pay new taxes implemented in 1934(USA), but still make a profit. Back then the cost/gal of fuel was pennies, so adding a full cent to the cost to cover those taxes was deemed a deal breaker, since sometimes it would be ½-¼ 1/10 the cost of the gallon.