It seems to me that, in many ways, we think about the 11+ in the wrong way.
It isn't an infallible measure of ability - although very low ability children will get lower scores than very high ability pupils, it isn't particularly good at genuinely ranking pupils in between. On another day, or with a different year's paperrs, the children admitted might be very different. It is also not independent of other variables - coaching, schooling, cultural norms - so what it produces is a result that is an amalgam of all of these.
It doesn't measure who would benefit, or be able to cope with, a 'grammar school' education, whatever that might be. It is not that in Kent 25% of pupils can copen with this type of education, whereas in a superselective area only 1% would be. I suspect that any child in the top 10% or so of ability, with supportive parents and a good work ethic, could cope perfectly well with a superselective grammar school environment.
What it is designed to do is provide a simple to mark and somewhat objective way of RATIONING a scarce resource - a limited number of places, in this case. It does this in a transparent way, and is adequately good at it - it separates from thousands of applicants a subset who are, with a few outlying exceptions, a decent fit for the school. That is all it NEEDS to do, from the school's point of view.
So arguments based around your child's ability are not really the point - yes, other children of that ability (measured in other ways, or on other days) and lower, will have been accepted by the school. Many others of equal and higher ability will not have been accepted. Many, many children who took the test, and many who didn't, could cope with, may perhaps even do slightly better than they otherwise might in, a grammar school environment. But your son, on this occasion, did not meet the rationing criteria.