The link to the Standardised score explanation, only shows an example illustration. The 79 shown in the example illustration relates to the raw score and is not the percentage, I'm afraid. This example could be for a maths or VR paper. Also different grammars have different tests. Some will give a VR paper with 85qs, whilst others may have papers with 100 or 80qs.
The scores would vary for different exams. It depends on the number of children sitting the exam, their ages and the average spread of scores across that test, as to how the standardisation would be done for each individual paper.
Here is another explanation from NFER, may make more sense.
NFER Standardised score explanation
More information re: Slough Grammars' admissions can be found on the Sticky at the top of this forum.
This thread has some info re:St B's, but it was when there were 3 tests to sit, so things may be slightly different now.
Slough Grammars' admissions