Under MacOS X, Apache's http.conf file is augmented by all files in the directory /private/etc/httpd/users/ Notice the last line of Apple's http.conf file:
Include /private/etc/httpd/users
That's intended to load http.conf customizations for individual users on your Mac. What it does, effectively, is append every file in the directory /private/etc/httpd/users/, in alphabetical order, to your /etc/httpd/httpd.conf file.