There are many different blends of gas, and refineries change with the weather (winter fuel mix evaporates faster than summer mix, and will not need summer emissions). Gas stations often choose their own mix of detergents/additives, but otherwise all fuel in a town comes from the same refinery with the same contents.

Most stations just go with the refinery default additive mix, which is typically pretty good. Some national brands go with a slightly better additive mix. Some national brands go with a cheaper additive package. You need to be a large chain to afford to pay a chemist to design an additive mix though, so the majority of stations don't.