Cody, actually, it can mean that the results are lower for some children.
Those with 'spiky' profiles - very able in Maths, for example, but didn't pass the 11+ due to difficulties in English - will not have their peers in the same school, and therefore may not be able to do e.g. the Further Maths they might have done had their peers been there.
It may also affect the teaching profiles - in this era of shortage of teachers, maths teachers interested in access to the teaching of the very able may no longer apply to teach in the 'non-grammar' school, and therefore the pupils remaining may no longer benefit from that particular teacher.