It's a somewhat subjective judgement.
Easiest to understand by looking at an extreme example.
Let's say that on the one hand a child has a compelling medical/social need to attend the catchment school, backed up by strong evidence from doctors and social workers. And let's say that on the other hand the school just about has a case - the PAN is 180, and there are 5 forms with 31 children and 1 with 30. but no really serious problems in terms of accommodation, staff shortages, numbers of children with challenging behaviour, numbers on special needs.
Would you be inclined to uphold the appeal?
It gets harder of course when the cases are more evenly balanced.