No one performs at the upper limit of their abilities on anything like a regular basis and it's basically a crapshoot whether your question and dialogue is actually able to test their limits. Most people, myself included, have a whole litany of things they need to perform well and doing it on-demand in a fundamentally stressful situation like an interview with a person you don't know is unlikely at best.

There's also a tendency for interviewer to choose questions from their personal expertise rather than something appropriate to the person or role.