Why should you be aware of an interviewer’s “hidden agenda”?

a. so you can find ways to address them in your answers
b. so you don’t fall into the interviewer’s trap with the agenda
c. so you don’t offend the interviewer by not knowing the agenda
d. so you arrive at the right time during the hidden agenda

What is implied in this sentence from mark twain's "the ? 1,000,000 bank-note"? i was puzzled, and wanted to discuss the matter a little further, but they didn't; so i took my leave, feeling hurt and insulted to be made the butt of what was apparently some kind of a practical joke, and yet obliged to put up with it, not being in circumstances to resent affronts from rich and strong folk.