The lion is the stronger animal

(Grammar Questions I Couldn't Answer)

This is another grammar question that comes not from a student but from a colleague.

The colleague was marking the student's homework.

The textbook had asked the question: "Which animal is stronger?"  (The question was accompanied by a picture of a lion and a sheep).

The intended correct answer was: "The lion is stronger."

But the student had written: "The lion is the stronger animal."

My colleague asked me what I thought.  "Yeah, it's correct," I said.  "I'd give him the point."

I would have been content to leave it at that, but my colleague wanted to pursue the matter further, and figure out exactly way the answer appeared to be grammatically correct, and yet still sounded slightly wrong to him.
So we batted around a couple of ideas.

My colleague suggested the sentence itself was non-native like.

I thought the sentence itself was absolutely fine, but perhaps sounded strange in this context.  After thinking about it for a bit, I suggested that possibly the problem was that the question was intended to elicit an adjective as the answer.  When the student responded with a noun phrase, it sounded slightly off to our ears because we were expecting an adjective phrase as the answer.

But I'll throw this one out to the Internet for a second opinion.  Did I explain this correctly?