loading libraries and models from within a model

[eluser]GSV Sleeper Service[/eluser]
What's the correct way to load libraries and (other) models from within a model?
The user guide is pretty vague about this.

a confusing example below

Code:

class Quotes_model extends Model {

    ...

    function save_quote()
    {
        $this->load->model('address_model'); //no error raised
        $this->address_model->get_primary_address(); //Fatal error: Call to a member function get_primary_address() on a non-object
        echo $this->session->userdata('example_session_var'); //works fine
        
        // CI get instance method below
        $CI =& get_instance();
        $CI->load->model('address_model');
        $CI->address_model->get_primary_address(); //this works fine
    }

}

so, why does '$this' in a model sometimes refer to the CI superclass (sessions, input etc), but in other cases I have to use get_instance()?