I think this is as simple as your first example being a default type Zend_Controller_Router_Route_Module
route that is normally relative.
Your second example is a named route so it needs to include the base url in order to be valid. I don't believe rewritten routes can be relative as the route may or may not refer to an actual location.
I've done some limited testing and I believe that if you set your baseUrl value the first example will also include the host address. So for consistency your urls will always be absolute.
//application.ini
resources.frontController.baseurl = http://www.domain.com
Hope this helps.