Exactly what Raoul has been saying, as Superman he's basically acting like himself, he's being Kal-el. As Clark he plays the mild-mannered mid western just to fit in.
He's more than just a human that was raised by Martha and Jonathan, he's also a Kryptonian with a noble alien legacy. It's only when he's Superman (Kal-EL) that he embdies everything he is - as Clark he only shows a fraction of himself, right down to his personality.