Raspberry123 age standardisation doesn't work on the basis of 'oh my child is summer born and really clever, so no summer born children are disadvantaged, they're just not genetically gifted and don't work hard enough'.
Age standardised scoring - which the OP is talking about - looks at large cohorts, maps trends, then adjusts scores.
It's much more complex than August born children 'being given extra marks' (this doesn't happen btw).
OP's central point, that her summer born child child is disadvantaged in regards being picked for the maths challenge as she is months younger than the children who scored one mark higher, which she probably would be able to do if she sat the test in a number of months time, but that isn't how selection works as the school choose the children who did best NOW, is a good one.
The related point, that it may not be a big deal in the grand scale of things, but it is to her daughter is an extremelyimportant one.
If you look at growth mindset research (which is substantial), this system could be seen to actively discourage summer born children from developing it as the 'not yet' is irrelevant as the criteria for not being selected is 'not now.'
The effect this has on individual children will of course vary, but it's fairly basic educational psychology that being rewarded (by being chosen) reinforces the child's decision to try for a test which will influence their future choices, self-perception and attitudes, while the reverse is also true.
I've always been an academic outlier, which gives me the intelligence to recognise that my experience is exactly that. It's not the 'norm' nor should it be used as a basis for formulating any sort of norm.