The octane rating is the average of two ratings, the "motor" octane rating and the "research" octane rating. Different engines prefer more of one or more of the other, although the average number is the same. Different gasolines from different refiners may have more of one rating and less of the other, with the same average number. You seem to have found what your engines like.
Even a non-refining marketer of gasoline may have a long term contract with a certain refiner, thus have the same product for long periods of time. This Gulf gasoline, by whomever refines it, seems to be good for your engines.
Do your engines really benefit from the high priced 93 octane gasoline? High octane gasoline is not better than low octane, it just has more resistance to preignition. If 87 octane runs well in the engines, figure your actual fuel cost per mile and see which gives you the lowest cost per mile. It is a very small number of engines that actually need high octane gasoline, except to produce that last 5% or whatever of max power.