It will be difficult for any school to cleanly top slice the brightest applicants, and some odd decisions will be inevitable.
This will be true of all schools. The problem with SPGS seems to be the attitude of some SPGS parents. One mother acted as if there had been a death in the family when I revealed that DD was going to an equally nice school nearby. She then proceeded to act - and I met her several times after that, as if I was not worth knowing. Another father decided to explain to me in depth how SPGS collected all the cleverest girls in London, without seeming to mind that he was clearly labeling my own child as less bright. (A couple of terms later, though, the same parents were moaning that the school pushed the girls too hard!)
It is very strange, and I sympathise with Nickmom.
Several of the girls we know that have gone there have had an awful lot of coaching before sitting the exams, with the names of good tutors being carefully passed around. Not getting in is really not an option for some. Boys have more choice, with Kings Wimbledon, St Pauls, Westminster, and boarding (Eton) all being seen as acceptable. I assume that once in, a girl needs to keep up, something which would not be so important in a school with a broader intake. One we know, was really high on the wait list but did not get a place, and is now nothing special in her fallback school, though a sense that girl and mother are slumming it a bit both socially and academically.
There are a number of good schools in West London, both private and state. Presumably any child who is being considered for a place at SPGS should thrive in other schools. At which point I wonder whether it is more fun to be in the top third of another school, and able to find your own level, or in the bottom third at SPGS and feel as if you are struggling. I suspect the first group will come closer to achieving their potential, and emerge perhaps with better social and emotional skills. I also wonder what the pressure is like once you are actually in the school. We turned down a place at the boys school for our son, in part because we met a mother who was able to list exactly where her son came in his year at Colet Court and happy to write off a couple of his peers as "not very bright". I don't think she was unique, and have heard others describe a system that produces some very anxious children. Her boy was in the same year that my son would have joined, and we did not want this for him.
Its hard though. These girls, at least if you listen to the parents, all seem to be so bright, musical, sporty, artistic and sophisticated. I am not sure what my rather ordinary DD makes of it all. I am glad she does not have to cope with it at school.