Do your local schools admit some children partially based on ability or do they just stream them once they've been offered a place?
This may be the difference. For example we have a comprehensive that admits just on distance but when all the children have been offered a place they then have the opportunity to sit a test for "Fast track" which is a completely separate stream. They have about 60 fast track places out of 180 and everyone else is put in mixed ability groups. Importantly though, the admission criteria is not based on ability at all. Hence this school can't be called partially selective.
Other partially selective schools admit a certain number based on their ability - it is part of their admission criteria.
Hope that helps