Wordpress Config file 403 Error - How?

I have a quick question that's driving me insane and would appreciate the assistance.

On a live wordpress setup, how does accessing /wp-config.php from a browser result in a 403 error when you can access all other PHP files fine froma webbrowser? The config.php file has a chmod that permits public read AND there's no mention of it in the corresponding .htaccess.

Just very curious as to how it works. Thanks!