It is difficult to understand the reasons behind it all.
The boy in question (he was in the same year as DS) is exceptionally bright.
My husband met the boys' father at the AFS last year and his father stated that the only reason he entered for the KEGs exam was because his ex -teacher at primary suggested it: so there was no 11+ training involved and no time...
My experience with DS in Y7 so far, is that you are competing with children who are very bright and are required to meet the very high standard that the school establishes.
All grammar schools are different and have their own standard. My husband is a grammar school teacher and although, he works in a good school.. the teaching at KECHB is generally outstanding.