| Refineries make hundreds of different fuel mixtures. Every state has different requirements, and of course within that there are several brands that further have requirements. (Learned from a great modern marvels or similar show on discovery all about oil)
BP is one of the few that makes their own fuel and aside from meeting individual state requirements is probably the most consistant, so I'm not surprised you had a good experience. I am surprised you managed to find that many BPs though!
One thing I do know is Mobile is nothing special. They're not part of the same clean alliances that the other brands like shell are. Yet they charge the highest prices, and people continue to line up there even with a cheaper station with potentially better fuel right next door!
For the most part I go to the cheapest place I can find. Around here they're all no-names and all full serve. As much as I hate full serve, I like it better than paying 25 cents more a gallon at the mobil down the street! In fact I did notice a pattern last year that I consistantly got worse milage off mobil. | 
