I have not primed a keg yet, generally I go with Sean T's method of set to serving and shake. However I always assumed that the reduced amount of priming sugar had to do with the reduced headspace per ounce. The volume of headspace in a 12 oz bottle divided by 12 is greater than that in a keg divided by... 640? But that's just an assumption I came up with to justify what Charlie P says in the book.