Just cause you buy fuel from a BP outlet doesn't mean it is BP fuel.
All BP fuel in Sydney comes out of the Shell refinery at Clyde. A little while ago all Shell premium petrol came from Melbourne. That may have changed because they did a lot of work at the refinery 6 months or so ago.
It not just these 2 though all the fuel companies do deals amoungst themselves. Mobil in Sydney for example is Caltex and interstate similar arrangements are done.