I think these dualities exist in all prefessions: what the public think you should know about, what you actually know about to do you job/research.

Example 1: I'm a computer scientist. I study algorithms, computational theory and logic. But I can't tell you why why some application (say MS Word) always freezes and how you can get that corrupt document back. Sometimes I think my mom wonders what she was spending her money on with my CS degree.

Example 2: My wife is a 4th year medical student. She is specializing in pediactrics. Her relatives think she should be able to diagnose all kinds of cardiac and geriatric conditions.

Sure experts in some fields do tend to know a lot about stuff in other fields. But not always. I think the general public needs some general education in job descriptions.