I cannot explain it, other than to say that as a writer, you've got to be nosy. Writers must be nosy about the people around them. Where else would we get material? Jane Austen was very social. I get the impression that she was, well, nosy. No doubt in a more or less impeccably polite and unobtrusive way, yet still, her eyes must have been always watching and her ears always listening. I believe she had such insight into characters because she studied people.
Knowing 101 obscure facts about Jane Austen might not make me a great writer, but studying people will help. What makes them tick? What causes them to make the decisions? What are they passionate about? Why? How do they react in different situations?
Here's your challenge today. Study a few people. Go to the park or the mall and sit on a bench and watch people. Use a newspaper or book to make it look like you're actually reading instead of spying. (Make sure it's right side up so you don't blow your cover.) Pick one of your subjects and write a journal entry from that person's point of view. Don't edit. Let the character tell you things about himself or herself that you didn't know. Keep these. You might end up using this character research at some point.
And here's to Jane Austen. Jane, you knew your stuff.