Writer vs Author

Whenever I mention to someone that I'm an author I don't always call myself that. Sometimes I say writer instead without really thinking about it. The two words are pretty interchangeable in my mind. However, I've noticed quite a difference in the reactions of those I tell based on which term I choose.

When I say I'm a writer or that I write it's usually met with a smile and nod of understanding, but no real interest. The listener usually moves on to the next topic of discussion without really acknowledging what I've just said. That's okay by me. To be honest, when I'm meeting or talking to someone as myself (Melissa rather than Missy) I'm perfectly okay with not discussing my writing. I don't push it on other people or even say much about it unless they show a genuine interest. I realize not many people are willing to admit they read the genres I write, so I give them an out by not making it a big part of our conversation.

On the flip side, when I say I'm an author the reaction is vastly different. Many times the person shows surprise and they ask about my books. How many have I written? What genre are they? Are they self-published? I answer the questions honestly and as succinctly as possible. What can I say, I'm not the best at self-promotion. I do offer a bookmark with my info if they seem to be genuinely interested rather than just being polite. 

I'm not bothered by either reaction. I just find it interesting that the two words, which mean essentially the same thing, garner such different reactions from people. I rarely say I'm a published author, and almost never that I'm multi-published, even though both are true. There's really no reason. Again, I'm not so great at self-promotion. 

Do the two terms have different meanings for you? If so, why and how?