The difference is if they studied the answer, but didn't grasp the material, they got information to pass the test (maybe), but probably didn't get useful information.
I guess if you stop at each point and ask 'what could go wrong here, and how would you debug it' and they answer that well, then they've gotten the information enough.
What is the difference? If they studied it, they now know?
Is it because as an interviewer, you are looking for knowledge by experience, not via book-learning?