They're two different scenarios.
A, is referring to what you'd do if you had an engine failure on takeoff at Brisbane. Assuming you planned that you'd make the decision to fly onward to Sydney OEI, how much fuel would you need?
B, is a different story. It assumes a normal takeoff and cruise, passing Sydney and continuing on towards Adelaide, but then, at the CP-OEI between Sydney and Adelaide, you then have an engine failure. Because you're between to suitable aerodromes, you have the choice of SY or AD to land at. How much fuel would you need if the engine failed at the CP-OEI between Sydney and Adelaide.
Also note, this is not saying that the CP for the entire BN-AD flight lies between Sydney and Adelaide (though it probably does). You would have two seperate CP-OEIs for this flight - one is between BN and SY - in the event of an engine failure before passing Sydney, you'd choose either BN or SY to land at - not Adelaide. Also, once you'd passed Sydney, the only options you'd consider is either turn around and go to Sydney, or continue to Adelaide - you wouldnt go all the way back to Brisbane, because that would by definition mean flying past Sydney, and therefore using more fuel than if you decided to land AT Sydney.