It's because of the falling birth rate. Medway only deems 25 per cent of the school population suitable for grammar school. As the total number of kids in the school year has shrunk, so the number of kids in the top 25 per cent has also dropped. Meanwhile the number of places at grammar schools has not dropped...so there have been places left empty.
I may not be explaining this very well. Here's a link to an article that explains it better:
http://www.telegraph.co.uk/education/ed ... desks.html
Chatham Boys Grammar also does not do as well on its results as the other grammars for boys so that's why its probably the least preferred choice. It's right next to Chatham South School too.