The major oil companies used to do fuel swaps for those places where they do not have refineries. So if there is she'll refinery only in an area the fuels come from it. They do have their own additive packages, and for fuels like 98 they tend to keep it to themselves. On one car forum I am on a few of them are tanker drivers, and I had to deal with these arrangements for work when the gst and fuel excise arrangements were introduced.
There is also a lot more imported fuels these days, which the independents use to stay price competitive.
But if you places that seem to be better, they may well be. With the st1100 some tanks were somehow better than others, but with Efi does not seem to matter as much, but e10 gives slightly less range wheras in the 11 I got equal best as must run a bit leaner with the carbs.
Now Wilcannia is a different matter! As Lionel...