The manager of the pumps at my local Costco explained their pricing procedure to me a few years ago. Each morning, he has to check the prices at all of the other gas stations in the same zip code as his station (I assume that he uses Gas Buddy, or something similar), and to price his gas 1 cent lower than the cheapest competitor–including the no-name stations.
As I’ve learned over the past couple of years, that leads to variations in Costco’s gas pricing. There is one Costco station about 12 miles East of my house that consistently charges ~13 cents per gallon more than the one closest to my house. Another Costco station about 20 miles South is almost always ~7 cents more per gallon than my local one. So, when you go to a Costco station, you can be pretty confident that it is the cheapest in that zip code, but it may not be cheaper than other Costco stations. Now that I have become aware of these patterns, I know which Costco to patronize fpr my usual fill-ups, and–luckily–it is the one adjacent to the Costco where I shop once each week.