The .9 is, of course, pretty meaningless since every oil company does it and it's usually so small that it's virtually impossible to see from a distance. Anyway, it's always there so you always ignore it. This is a wonderful example of just how quantified our lives are and how certain numbers are significant and other are not.
Perhaps, tomorrow, the price will have risen to 3.99.99!
The gas prices in the picture above are actually petrol/diesel prices in the UK. They are in pence and are for 1 liter of petrol. A liter is equal to about .26 US gallons so there are around 4 liters in a gallon. The rate of exchange between dollars and pounds sterling is around 1.6 dollars per pound. So the cost of petrol in gas equivalent gallons is roughly 4 x 1.48 x 1.6 which works out to around $9.47 a gallon.
Somehow, $9.46.9 doesn't seem to make a whole lot of difference.
What does make a difference is that 70% of cars in the UK are diesel and get about 70 mpg and places are much closer together so you don't have to drive so far.