Please let me know if I need to rephrase, I'm not very good at explaining things but I am pretty dang sure I know how this puzzle works. If you had a Zoombini without smooth hair or sleepy eyes but with a blue nose or rollerblades, you could know for sure which trait is the final one that causes the allergy and is only allowed onto the exclusive side, which is the bottom one in the scenario you got. The third trait would be a nose or feet (I've never seen two hair traits, or two eye traits, etc., on this difficulty) and my only guesses are that it's blue noses or rollerblades, since those are the only ones that don't show up on the top cliff. Notice how none of these were on the top side, but everything with either of those traits were rejected onto the bottom side.

(In any given puzzle you don't know which accepts and which rejects.) The bottom side is then exclusive to these three traits, two of which are smooth hair and sleepy eyes. But then it is allergic to three traits, which the bottom cliff accepts. The top cliff was the 'base' which accepted Zoombinis by default. In the puzzle you provided, you can only discern two of the three traits.

One is the acceptor, one is the rejector (the 'allergic' one). I don't think it worked this way in the original, but I am almost positive after doing this a lot that one cliff accepts only three single traits of three different features, and the other rejects those Zoombinis.