My brother once asked an airport rep the purpose of those seemingly silly questions. He was told that the purpose was not to elicit useful information, but to preclude any alibi in the event dangerous items were found by other means. In other words, if you were found to have weapons, explosives, whatever, you couldn't defend yourself by saying, "Gee, I left my bags alone for a while, someone else must have put them there." The series of questions is designed to eliminate potential excuses, not provoke a confession.
I suspected as much. But it might clue-in a few people as well.