Beg, steal, borrow, buy?

I've been looking at the lending and buying figures for the last decade, published by the Public Lending Right people, and some very interesting things emerge. First and foremost I was pleasantly surprised to learn that more books were borrowed than bought, despite all the 3 for 2's and loss leaders in the supermarkets. Also, the authors borrowed were very different from the authors bought - JK Rowling sold easily the most books, but was only 96th in the borrowing lists. Yet another children's writer, Jacqueline Wilson, was high up in both charts. What makes a book buyable or borrowable?
The bestlending list is split between "traditional" authors such as Catherine Cookson, and writing for children such as Goosebumps and Mick Inkpen. Does this mean that we get little old ladies and kids and nothing in between?
The lending figures also reveal a population who cheat at cookery (with the help of Delia) write poetry (with the aid of Stephen Fry), holiday in France, and read about war, atheism and cricket. The buying figures, on the other hand, show a liking for Grail quests, vampires, cup cakes, reality TV, and the Caribbean. This would seem to suggest that two completely different sets of people buy and borrow books, but this a clearly not the case - a book lover is a book lover, and will get their fix wherever they can, and indeed will often donate books after they have read them (thank you, thank you, thank you).
So why borrow Delia but buy Jamie?
Why buy Terry Pratchett but borrow James Patterson?
Why borrow war books, but buy Jade Goody?
I have no answers, but will be eyeing my borrowers keenly to see what they have in their shopping bags.