If I were to guess, I'd say that most of the mileage differences that folks are noticing probably come down to differences in ethanol content. Ethanol has less energy than actual gasoline. Most stations have stickers on the pumps that say "contains up to 10% ethanol." That percentage changes seasonally, and then you have some stations that don't sell you ethanol because there's enough people out there who don't want the ethanol in there killing the mileage.
CARB bans certain gasoline additives like MTBE that are normally added at the refinery to raise octane.