Also, any test that you ace has not completely characterized your actual skill level; all it tells you is that you're above the skill range measured by the test. You might be a little bit above, or far above, that upper bound. Maybe they've decided to have one really hard question that it's unlikely anyone will answer correctly, but if they get someone who can answer it then they know they've found someone really suited for the job.

Or it could be a mistake, and it was just bad luck that your friend got an interviewer who doesn't know how to interview, and asked a question that about a subject that they're intimately familiar with, but is too hard compared to what is asked by other interviewers.