Well more boost on the same engine increases the potential to burn more fuel, you will then burn more fuel if you use that potential. But you are right, for the same performance on a car of similar weight why should one burn more fuel than the other? The first thing that comes to mind is that the ST having a "sporty" orientation, it gives a natural invitation to be driven harder. If you go out for a test dive in a Focus ST aren't you going to "floor" it once or twice to see what it's got? This may not be the case with everyone driving the Volvo.
Additionally, with blown engines consumption can vary by a substantial amount and I see this with my Escort - if I manage to drive sensibly and keep the speed down I can reduce consumption by a quarter than when I drive "normally". Put your foot down and it quickly goes up by a quarter from "normal".
The bottom line IMO is that if you have the potential to use the power, you are invariably going to use it...