It sounds like the INS officer was trying to provoke a response from you. One technique they use to spot smugglers, illegal immigrants, etc, is to do something slightly out of the ordinary, which will cause the innocent person to be a little bit confounded, but will cause a guilty conscience to start displaying behavior like sweating, shifty eyes, stammering, muscle tension, etc. If the officer can provoke a such a response, they know to search and question you more carefully.

I've had the INS ask me some pretty off-the-wall questions on occasion.