Hi Jinx- I'm sorry, I wasn't ignoring you, I hadn't realised this thread was still going on (I tend to be a threadkiller
)
As CecilyP stated- if a GS takes the top 5% (or even 25%) then that patently does not include average children, as average children would be around the 50th percentile.
I wasn't saying your DD didn't deserve her place, I'm saying it's highly unlikely she is 'average'. 
I am pleased that the GS in your area take children from all backgrounds, but the GS in my area do not- mainly because there are many bright children whose parents have not got the skills themselves to assist with preparation, or do not realise any preparation for entrance is required, or do not have the means to pay for someone to do this preparation. Also because those with money (including those with children in independent school) pay a lot for tutoring to ensure their children pass the entrance examination. (as you point out- children who are coached can gain entrance even though they are not outstandingly brilliant)
I live in an area with fairly high levels of adult illiteracy, of English as an additional language (often not 2nd language, but 3rd or 4th language in fact).
You would be amazed at how many parents in my area are actually not equipped to support their children to GCSE level. There are also many parents who believe that it is schools' job alone to educate children, not their's.
I do know what I'm talking about when it comes to GS in my authority btw. Pass rates have increased dramatically here over the past 10 years, resulting in a raising of the score required to gain entrance (it was hovering around 300, 305 10-12 years ago, now you'd be lucky to get in with 340, most have 360 and higher).
Also- with regards to the background of children getting in- in my LA the percentage of children on FSM in the whole LA is 34%, whereas in the GS it is around 2.5%. This is a very strong indicator that children from humble backgrounds just do not get in.
Also regarding the ethnic backgrounds of children gaining entrance- children of Indian ethinicity are over-represented, and children of Bangladeshi ethnicity are under-represented, taking into account the relative numbers of children in each of those groups. Again, in the area I live, people of Bangladeshi family background are far more likely to be unemployed or a low-earner than people of Indian ethnicity. I am not making a judgement on the reasons for this- just stating the facts so you can see where I'm coming from I hope.