OP, this is astonishing.
If you are confident your child will be fine then go for it. With an older daughter in the school you know what the demands are and will have a good sense about whether your daughter can manage.
In our case the Prep school tried to tell us that our daughter was not academic and would not be able to cope in a selective London indie. Her CAT score was awful as was her 11+ result. Despite this she did get into a selective indie ahead of girls who were seen as shoe-ins. We were not worried as to us she seemed as capable as her supposedly academic older brother, just different.
Its fine. She is towards the top of her cohort, and probably doing much better than a good proportion at the grammar. (Indeed I wonder how good the grammar actually is. Given how selective they are, their results in comparison are not stellar.) She just was not ready at 11 and has got her act together since.
I would be tempted to try to get this message across. My assumption is that the grammar are concerned that a girl 30 marks below will not cope and are telling their education authority this. The worst that could happen is that she gets in but the school have already written her off. I would also try to ensure that she will cope. Perhaps some help with core skills such as maths or English before she starts if she is weak in either. Or help with the first year exams, as this is when schools really decide who is struggling. (I took a week off work and helped my daughter revise. Great as I also learnt that aural revision was much more effective for dyslexics.)
If however you are concerned that she might bump along the bottom, I would try to parlay the error into agreement that she goes to her second choice school. Grammars are not necessarily better, just more selective, and being at the bottom, knowing you only got in in error, would be awful.
Dont tell people. Girls can be cruel and it is very possible that insecure girls will remind your daughter that she was lucky to get in.