No idea. I am as much of an idiot as you are. But questions of this kind were supposed to detect candidates who were able to think.

There's no actual reason. The test is to see if you can invent convincing-sounding rationalisations on the spot. Or, in practice, to see if you've read any books about interviewing.

The interviewer is an idiot. Manhole covers could have all different shapes: round, square, rectangular and other.