A person seems smart. She is an expert in her field and can speak intelligently about her field for hours/days. Does that mean she is smart?
The thing to remember is someone might be smart in one field but not in other fields. A neurosurgeon might be extremely smart about the brain but have no clue about plumbing or electricity. An accountant might be brilliant with a spread sheet but not know how to turn on a stove.
Someone being smart (or not) typically is related to the particular topic. A person can be smart in one area and appear quite dumb (or naive) in other areas. The polymaths of the word are few and far between.
Have a great day!