Also, it is a matter of state of mind and mood - you might not believe you understand the concept because you do have bigger picture, but you might also understand it as intended by the test, but not believe that you yourself understands. We have an image of ourselves
It's getting a sense of some kind of measurement of how it is written in the brain.
It's not understanding : brain != you. and not limited to the concept. For example, it doesn't take into account how well you "feel" about the concept. People are not computing machines per se. People are very emotional for one thing and without any research everybody knows that your current mood and state of mind influences a lot. Two people with the same understanding could fire different brain patterns, perhaps.
1. US only
2. It seems that "living" does not include almost nothing beyond the immediate living requirements and it does not take into account times you cannot work, can't find job, or simply the fact that a person is a little more than her capacity to be a machine for someone.