They are much larger schools, more centralised.
DD started out in a Kent grammar school. Her school only took girls from the top 5-10% of 11 plus results. However there were two girls' grammar schools in the town, and two boys'. The other girls' grammar took girls from the top 15%. This of course depended on places - I too was shocked when I found out that maybe some in the top 15% would not be allocated a place.
The top boys' grammar took from the top 10%, the other, from the top 20% because although there were similar places available, the boys did not do as well as the girls so boys who didn't do as well as girls nevertheless were allocated places.
All of this was explained to me by the headmistress of DD's next grammar school, which was in Lincolnshire. There, it was the only grammar school around for miles, which is the case all over Lincs as the population is so sparse. She told me that depending on how well the pupils did they had enough spaces for between the top 15% to 20%, so that DD would perhaps notice that the pupils may not seem as quick-witted as those she was used to! I looked at her and she said, well, she had just moved up from a grammar school in reading and she certainly noticed it!