[musingsole]

You're missing the part where the interviewer is changing with each interview (and the rest of their life in general). They phrase the problem differently, get anxious as the interviewee approaches different parts of the task. So, the interviewer thinks they're giving the same test and can fairly compare the outcomes. The GP's point is that's just not true

Whether it works out to be a harder or easier interview over time because of these changes probably depends on the individual interviewer.