[simsla]

Good / bad / unclassified.

It makes sense for unclassified to smell worse than good, and it'd probably be the biggest category by a long stretch.

(Pure speculation.)

[big-and-small]

And even good / bad is sometimes subjective and brain can adjust to it depend on whatever you like the taste for instance. Tell you this as a big fan of durian. Since there a lot of chemicals responsible for smell brain override reaction to fruit once you love the taste.