I know it’s not strictly on topic but it’s not irrelevant. It shows you that an incredibly bright person who would have been at the top end of a grammar couldn’t pass the test, despite attending teacher-led preparation classes at school. DC1 actually took two 11+ exams for two different local authorities so it wasn’t a case of having an off day.
To me it is an example of what a poor selection tool the 11+ is. Presumably this is one of the reasons why most of the country abandoned it.
I went to a grammar myself. We all passed the 11+. There were several in my class who should not have done. My best friend from primary did not pass and is one of the most intelligent people I have ever met. (I’m still in regular contact with her).