Hunger as a word has a negative connotation. Having a low % fill in a bar title "hunger level" intuitively makes you think "Low is bad". It would make more sense to the layman to title the concept "overstuffed level", or perhaps "gluttony level". Both of those concepts are negative when you have the max of it, so it's intuitive that having a high amount of it is bad.
Ok... so I kinda butchered that description and I'm bulleting it XD
Filling hunger bar sounds like a good thing. Filling an overstuffed bar or a gluttony bar doesn't really.