Not sure you do much grocery shopping. Some of them do exactly this.
Here is the deal that most folks don't realize, and it makes them mad. If a dealer has a half tank (his big tank) full of gas he paid $3.00 a gallon for, and is selling it for $3.50 (I have no clue, I'm just making up examples), and all of a sudden the retail price jumps to $4.00 and the wholesale price to $3.50, he can't keep selling his gas for the same old $3.50 or he won't have enough money to buy the next tank load when he orders it. Yet when he raises his price on existing inventory, folks go nuts and say he is 'price gouging', when he is not. He's trying to ensure he has enough capital to buy the next product at the new wholesale price.
And most folks forget that a month or two ago, it was $2.90 a gallon and it fluctuated daily then too..........but no one cared about that, that was fine.
There's only one reason that folks get so mad about gasoline prices, and not milk, or sugar, or cokes, or beef, or cheese, or silver, or stocks...............and that's because they love what it does for them and they use it every single day. And almost no one will take the time to actually try and learn how gas prices are calculated and how this market works. They just whine. A simple Google of "How gas is priced" would be an eye opener for most Americans. And Europe is paying about four times what we do.