Imagine two apples at a store. One is prices at $3.99, the other is $4.00. Even though the prices are essentially the same, people quickly glancing at them will treat the $3.99 apple as significantly cheaper. People are bad at understanding numbers, especially when they are making quick decisions.
People seem to be much more sensitive to the price of gas. One extra penny is more likely to be noticed. Gas stations instead shave a tenth of a penny off the price to make it feel a whole penny cheaper. People see 3.99 and 3.999/10 as the same price at a glance.
Most of a penny may not seem like a lot, but when the US consumes over 100 billion gallons in a year it adds up.