It's not an interrogation. It's usually two or three questions. The answers really don't matter. They are trying to get a feel for the person who has just presented them with a document that may or may not be legit.
It is a quick way for them to verify that you are who and what you claim to be. If you won't let them do it the easy way (and, the gentleman is right; he doesn't have to), they can do it the hard way.