Around here they are 'priority admissions areas' - and schools are often full before the edge of the priority area is reached. So the 'area' is a fixed one, drawn on a map, but what really matters is the last distance admitted for non-siblings...in most cases.
However, it is possible to be within the last distance admitted, but outside the priority admissions area, as these are seldom circular!
E.g. if you live 1 km North of school A, and the priority admissions area boundary falls between you and the school, at 980m from the school, you won't get admitted as 'in priority admissions area', and will be 2 criteria lower, even if the last distance admitted within the priority admissions area is 1050 metres.
However if you live 1 km South, and the priority admissions area boundary falls 1100 m from the school in this direction and the last distance admitted is 1050 m, you will get in as 'in priority admissions area' and within last distance admitted.
And as others will have said, last admitted distances can go down as well as up - more frequently down, for popular schools.