sprintf not having right default value

Default value never gets the right value assigned.

function feeds_tamper_sprintf_form($importer, $element_key, $settings) {
  $form = array();
  $form['format'] = array(
    '#type' => 'textfield',
    '#title' => t('Format'),
    '#default_value' => '%s',
    '#size' => 60,
    '#maxlength' => 128,
    '#required' => FALSE,
    '#description' => t('See the <a href="http://www.php.net/manual/en/function.sprintf.php">sprintf</a> documentation for more details.'),
  );
  return $form;
}