In Community Server there is an available option for users to remove www. from address of their sites. When you enable this (enabed by default), it redirects all requests to www.site.com ro site.com.
There is a .org site that I can't remember its Url now and there you can find some information about benefits of this. I personally love this feature because lets my visitors to have shorter and constant pernalinks.
However, using this configuration node you can enable or disable this feature and change its behavior. Available options are described in XML comments.