yeah pretty much all come from the same refinery (shell and mobil here in melb) to which different additives are added for different brand fuel. I believe BP is up in either SA/QLD/NSW (one of them) so the fuel is usually refined from the closest refinery then formulated to BP/Mobil/Shell/Caltex standards and transported to the servo.

IMO BP Ultimate and mobil are very similar
I don't use Shell and Caltex too much shit in shell's vpower and Caltex gave mum's car several problems from dirty fuel (more servo's fault i reckon for dirty tanks)