Animal difficult used to vary based on quality. Anyone remember the old animal levels where you could see a L1 bear and a L10 fox? Ok, I don't recall ever seeing a lvl 10 fox or L1 bear, but do recall seeing some L7 or 8 bears and such during w3. Animals were tied to civilization levels, and big villages and a lot of player activity tended to suppress the animal level, and thus difficulty and quality.
It's an option to go back to, but it wasn't popular then IIRC, and it wouldnt be popular now.