Hi Rongway
As cats get more sleepy they get automatically a little less happy than when they have plenty of energy.
Perhaps this explains a bit why your cat's happiness is fluctuating.
Also the amount of hunger the cat may eventually have effects it's hapiness but this doesn't seem to be the case I think, so I wouldn't worry too much.
Cuddles make cats always extra happy and so does milk and 92% or 90% happiness means you have a very happy and satisfied kitty :-)
