The charts are quite confusing! However if you look at Rathmore and Lumen Christi college Derry you will see that they have only a green entry which means they take in only grade A's. This kind of makes it a little clearer...I hope!
http://sluggerotoole.com/2011/09/03/grammar-school-intake-hoovering-up-pupils-at-the-expense-of-local-secondary-schools/
http://sluggerotoole.com/2011/09/03/grammar-school-intake-hoovering-up-pupils-at-the-expense-of-local-secondary-schools/