You have already touched on it, @Capricancer1: StAs actively considers GPA and ECs in a way that is different from the vast majority of UK unis- but one that is familiar to US applicants (and their advisors). From the pov of many US applicants that actually makes the application easier, not more difficult, b/c it is more familiar. You see that in the many posts every year by CC of students considering the UK who trying to get their heads around why that hard-won GPA or all those community service hours being largely irrelevant.
St As were one of the inaugural international unis to sign on with the the Common App when it went international (obviously more have joined since). While lots of UK unis welcome US students (& their $$), St As invested in getting to know the US market, from college fairs to marketing. That has extended now to helping students with applications to US grad schools- afaik, St As is unique in the UK in that.
Even in the two competitive urban markets that I know best (where there are numerous high-flyer independent schools), the GCs are most likely to know (and be comfortable with) St As. It is pretty impressive how successfully St As has spread the word (obviously the royal connection didn’t hurt in that regard).