Gas Prices

No I'm not going to bitch about gas prices, although we probably all could. What I'm going to bitch about is the variation in gas prices on my way to work. I live about 8 miles from the office and I drive by 8 different gas stations on the way to work. Here are the prices on the cheapest level of gas this morning.

2.09
2.09
2.21
2.14
2.19
2.11
2.19
2.20

The two $2.09 are right across the street from each other which obviously is why they're the lowest and the same price. However the $2.21 is less than a quarter mile from them, you can see both of them from it and vice versa. They are on the north side of the interstate and $2.21 is on the south side but only an interstate exit separates them. The $2.11 and $2.19 near the bottom of the list are right next to each other as well.

I don't understand a 5% difference in prices when you are right next to each other.