Girls sit the test. If they pass (achieve the qualifying score) then they are eligible for admission.
If more girls pass than there are places, then places are awarded according to the admission criteria. First come looked after children, then those who live in catchment (the 8 miles radius circle) with sisters already at the school.
Then comes those who live in catchment with places awarded by distance from the school. So the girl who lives the nearest gets the first place, the second nearest gets the second...and so on. So it is entirely possible to be within the 8 miles, pass, but still not get a place because enough people all passed who live closer than you.
Doesn't matter how high the scores are, as long as they score above the qualifying score they are treated the same. So in your example, if both passed, then the girl living at 6 miles out will get in before the girl living at 7 miles.
I've posted on the local site - the furthest offer last year was 5.42 miles from the school (that's for girls who are not looked after or have a sibling) so the 8 mile catchment area is meaningless for most girls - you probably need to be no further than about 5 miles from the school.
As others have said, it makes no difference if your DD gets full marks (423) or the pass mark (334).