I'm not sure tbh - as I started off on LL, I always advise my clients to drink 4 litres cos 2 1/2 doesn't seem enough to me personally!
Maybe CD will change their guidelines at some point but they do state that 2 1/2 litres is the minimum you should drink. Obviously, the more water you drink, the greater your losses (up to a limit of about 7-10 litres a day, depending on your build and height - as too much water can be harmful!)
I expect the reason CD only advise a minimum of 4pts is so it doesn't put some people off who feel extra daunted at such a large water intake. At the training day it was said that 4ltrs is what most counsellors will encourage.