Hello Kent Mum. As I understand it, some Kent grammars have an admission policy whereby places are offered on the basis of "rank order" in the exam (after sibling offers etc). These include most of the grammars in Tonbridge . At these schools, therefore, children may need to obtain scores significantly above the Kent pass mark in order to get a place.
The Maidstone grammars, however, use a different system. They simply look at whether a child has passed or failed and then, if they are oversubscribed, allocate places to children who passed on the basis of distance from the school to the child's home. A child who scrapes a pass but lives a mile from the school will therefore take priority over a child with full marks in every paper who lives two miles away. If you live in an outlying village, therefore, it can be impossible to get a place at an oversubscribed grammar even if you are technically "in catchment" and you ace every test. This was a problem at Invicta grammar a few years ago, when kids from our village couldn't get in. I'm not sure what the situation is with the boys' schools.
For anyone that is interested, admission criteria for each school are set out in the county admissions booklet which comes out each September (and is sent to all year 6 pupils, at least in the state sector- I'm not sure about private). The 2007 version is available online at www.kent.gov.uk
(follow the links through the schools pages).