The schools have always said being in the catchment does NOT guarantee a place for your child.They go by distance - starting with those closest to the school. Once their classes are full, the child maybe placed on waiting list (in the hope that those closer to the school receive better offer elsewhere).
Eg. this has been Cheltenham Girls policy for as long as I can remember:
One of the key factors in determining priority for placement is proximity to the school. However, it is possible that students living within our geographical enrolment zone may not be successful in gaining a place because the enrolment ceiling has already been reached. In this case, students may be placed on a wait list. For further details please refer to our Enrolment Policy.