Someone I know with experience of this said that aggression can contribute to a sufferer being awarded CHC but then if some one is medicated to overcome the aggression then it can be taken away.
My mum was aggressive in the middle stages, but this passed and she became very calm and compliant, without any medication. That is often part and parcel of the progression of AD.
So I guess if this happens to someone who was awarded CHC, they might say that once the aggressive stage has naturally passed, then that particular health need no longer warrants the previous high score.
Similarly if they were exhibiting unpredictable behaviour, lashing out at others etc, then once their decline in mobility (again a natural part of AD) restricts the impact it has, then I could see it scoring less then too.
I'm not professing to be any sort of expert, btw, or defending the system - I'm just trying to make sense of the anecdotal evidence we hear.