Why $params gets the values from configuration file already?

I have this is a piece of code which is part of a custom library for CI 3.1:

class NavigationMenu
{
    protected $CI;

    public function __construct($params = ['config' => 'navigation'])
    {
        $this->CI =& get_instance();
        $this->CI->load->helper('url');  
        $this->CI->config->load($params['config'], true);
        $this->CI->load->model('nav_model', 'nav');
    }
    ....
}

The file 'navigation.php` have the following code:

$config['navigation_open']          = '<ul class="nav">';
$config['navigation_close']         = '</ul>';
$config['item_open']                = '<li>';
$config['item_open_active_class']   = 'active';

I have notice that $params passed to the constructor reads as:

array (size=10)
  'navigation_open' => string '<ul class="nav">' (length=16)
  'navigation_close' => string '</ul>' (length=5)
  'item_open' => string '<li>' (length=4)
  'item_open_active_class' => string 'active' (length=6)

Why? Is this a default behavior from CI? From PHP constructor and I am not aware of it? Or something is wrong in my code and I am not seeing it?