blacktop™ wrote:I guess it could be to do with the crappy drive by wire system...?
^^ This.
Older Subaru's have a cable-operated throttle with a direct physical connection between the accelerator pedal and the throttle, but newer ones run what's called "drive by wire" - basically, the computer reads the input you provide with the accelerator pedal, considers it with a whole bunch of other parameters, makes a decision how much throttle opening it should create/change, and then executes that change.
There's not only lag in this system (and it's very noticeable from idle when you prod the loud pedal), but you may also not get the change in throttle position that you expected - you'll then adjust the accelerator pedal more, the computer will recalculate, and you might get what you wanted. Or you might not.
It's possible, through tuning, to improve the behaviour, but never to eliminate it entirely.