Exams and pinholes.

Pinholes are peculiar. From the distance they don't allow you to see anything, but come closure and you can see the world.

Same are exams. From a distant you feel you know everything in a subject. But as exams come closure, you see where you actually stand.

So the more exams you give, the closure to the truth you will be. The honest your assessment will be.

The most important thing is, it just doesn't apply to exams only.