Currently, some libraries in system_library() are explicitly put into group JS_LIBRARY, while some others have no 'group' value assigned. For example, jQuery UI Core is explicitly put into group JS_LIBRARY, whereas jQuery UI Accordion has no group value set. This makes one think accordion is put in group JS_DEFAULT, however, this is not the case, as everything added with drupal_add_library() is automatically put in group JS_LIBRARY by this line of code: $added[$module][$name] = drupal_process_attached($elements, JS_LIBRARY, TRUE, $every_page);
To clean up this incomprehensible mess, we should either
* specify a 'group' value JS_LIBRARY for every library (instead of not specifying it at all), or
* remove the 'group' value whenever it matches the default value.