It is very difficult to tell you exactly how the standardised scores are calculated as each 11+ region will be different and in some places (like where I am in Trafford) there are different types of 11+ exams and some of the grammar schools standardise internally whilst others standardise externally – so there’s no ‘set’ method.
However, there is a general rule (like the one applied to the National Curriculum SATs Tests) that older children in a school year will usually receive less of the marks awarded for age standardisation. This means that children with a birthday in September will have a slightly lower standardised score than the younger children in the same year with birthdays for example in June, July & August – it will be on a sliding scale.
This adjustment makes it fairer to all pupils. One of my ex-pupils had his birthday on 31st August (= only just in that school year) – so he received the maximum age-related standardised score, but in the event he passed easily anyway as he was clever enough for a grammar school.
Hopefully that explains things a little for you.