Code passes tests, but solution not approved?

First off, I'm loving Exercism. I think it's a fantastic concept and you guys have made it painless to participate.

I've been enjoying the Elixir course so far. Trying to get my head around Elixir has been daunting, but setting these problems to solve has really helped me start thinking functionally.

I was curious about the philosophy and "grading" standards for submitting solutions. I've submitted solutions for a couple of exercises that have both passed all of their tests. Yet, the mentors have pushed back and asked for a different solution. In one case, my solution is more robust than required and accounts for a potential error case inherent in the setup, and in the other case, the mentor has suggested a hint toward a solution that would have never occurred to me in a million years of programming.

My question is this: my code passes the test, shouldn't it pass the exercise as well? I don't have a problem with considering other solutions, but to fail a solution just because it doesn't pass the tests in the expected way doesn't strike me as the correct way to grade these exercises.

Perhaps this falls into the debate between coding strategy and approach?