I enter this conversation with some trepidation, given who I am employed by I guess I have a bias!
I work in BP's IT dept, not Marketing or Engineering or Pricing.... So this is only what I understand of the process. I am no expert.
Wattio is correct, MOST (not all) raw fuel in a given state does come from a particular refinery. eg: in Vic Shell Geelong and Mobil Altona. The difference is the different oil companies add different combinations of additives to their fuel at their own terminals. While BP and Mobil both sell 95 Octane (premium) the chemical composition is slightly different according to the preference of that company.
With the 98 octane fuels, all BP's Ultimate comes from Kwinana, Caltex's equivalent comes from Kurnell etc. I don't understand the technical part of it, but there is an extra process involved in producing the 98 octane fuels, something that cannot be done at the terminal, only at the refinery. Again each oil company has their own process and specification. So BP can only supply so much Ultimate, Shell so much Optimax etc as they have to truck it around the country. My understanding is only the higher volume or higher profile sites have the Ultimate fuel (at least at this stage).
Often there is a price differential between 95 and 98 octane when they are available at the same site. So the customer then has more choice at some sites which one they want to buy.