node drupal_rendered twice on node page

When displaying a node page (node/%nid), drupal_render is run twice on the content of the node :
- Once in template_preprocess_page() : $variables[$region_key] = drupal_render($variables['page'][$region_key]) for $region_key == 'content'
- Once in template_preprocess_node() : $variables['content'] = drupal_render($node->content);