example.com

and if you decide that you are going to publish and market your domain as

www.example.com

you setup a subdomain in your DNS as well, a 'www' subdomain. Whichever you decide, you typically want to redirect traffic sent to the non-authority-site to the other, the one you have determined to be the authority or "primary" site. Have a closer look at your DNS zone file entries as well as your server for specifics. Looks to me like the 'www' is the decidedly primary site.