Why is Sanji addressing Nami with “San” and Robin with “Chan”?

D: Oda-Sensei, After I started watching One Piece, I thought of it all the time, but why is Sanji addressing Nami with “San“ and Robin with“Chan”? Yet it is Robin who’s the elder!! Is there any deep meaning behind it!?

O: Well I actually didn’t think anything of it. Before there was a staff member in our workplace who asked us something interesting. The wish to treat a woman older than you as if she was younger, and a woman younger than you as if she was more of an adult….Is Sanji acquainted with this feeling? I replied, “O, Ofcourse. That is right”