Was going to reply that I remembered hearing/reading that Shell was one of the few companies that only sold its own gasoline at their stations, but then I stumbled across this article:
http://blogs.platts.com/2010/08/23/unbranded_vs_br/
Seems like the issue could be down to whatever additives were (or were not) added. Like a previous poster said, there are definitely winter and geographic blends as well. If you’re really interested search for RBOB + whatever region/brand/season/etc. you’re interested in. With a little digging you should be able to find the specifications for gas in your area, but I’m not sure how useful the information would be from a tuning perspective.