Agreed - the answer to this question is really the result of how much one thinks one knows. Because a little knowledge can result in false confidence, those that rate highly will fall into two categories: Those that actually know, and those that don't.

If the interviewer can distinguish between these two categories, then what value does the question add? If the interviewer cannot and is relying on the answer to this question (even partially), then it's an example of relying on a false signal.

Whenever I'm asked this question, I always respond with the following questions:

1) What does a "10" mean?

2) What does a "1" mean?

3) What's the difference between 5, 6 and 7?

4) (Optional, this could easily come across as being snarky) What's the range of levels in your team/org/dept for this metric?