As a tutor and a parent, I find 11plus exams and Sats levels are two different ball games.
I have pupils who perform very well, day to day, in the classroom situation whose parents (understandably) expect them to pass the 11 plus because of this, but the moment you put non-verbal into the mix all bets are off.
Specifically, I find some very creative children (who can write wonderful essays) will struggle with the sheer logical discipline of non-verbal. With these children, I have to ingrain rules and processes so they almost switch off this creative side and adhere to a rigid step by step method.
Even a typical verbal reasoning paper requires a fairly logical approach.
I tutor for both an NFER-styple paper and a University of Durham designed exam. I find the two exams favour quite different types of children, and results bear this out.
As to Sats levels, my own child received 4's in both her English papers on Year 6 despite having a fairly high standardised score (71 above a pass) in the above-mentioned NFER-style exam. She was able to do this because the only part of the exam that was remotely English-based was the verbal reasoning and, even then, that just required a child who can spell and has some vocabulary. The fact she struggles with comprehensions (can't link questions with answers) and hates any form of composition was not relevant.
Do I expect my child to be near the top of the school? No, I don't. After all, no one is ever going to notice again that she is a whizz at non-verbal; meanwhile comprehension problems will affect most subjects. As I tell parents, the entrance exam is one thing, doing well at grammar school quite something else.
Over the years I have known of several children who were high-flying at their primary school, failed the exam, then continued to be high-flying at their destination school (whether it was the local sink comp. or the prestigious private they were squeezed into at the last moment). What I suspect happens in such children's cases is that, because they appear so able at school, their teachers often give parents the impression that grammar school is 'in the bag' and possibly less dedicated preparation goes on.
The grammar school system was largely discredited because it was felt unfair to categorise and decide a child's future based on one exam at eleven. Your clever children are proof of this unfairness, but I suspect it is harder for you to bear in that, on a daily basis, you encounter children much less able than yours when you're teaching.