It does seem a bit arbitrary and I wouldn't mind betting some of it depends on how much there is left in the pot, and what time of year it is. Or am I being too cynical?
Just because you are being cynical doesn't mean you are wrong! It shouldn't depend on how much is in the pot - if someone meets the criteria then they can't 'officially' be refused on the grounds of cost - but I think they frequently are.
I have even heard the opionion that you have more chance of getting awarded CH after the person has died, as the PCT know exactly h0ow much it will cost them. They have no way of knowing how much the bill will ultimately be when someone is still alive. Having said that, people who are awarded CH sometimes have it withdrawn on review which must be devastating when you have faced a huge battle to secure the funding in the first place.