Why does the title callback of /user produce the title for /user/register?
For the user/register title to be changed in a theme:

function mytheme_preprocess_page(&amp;$variables) {
  // Only act when the first part of the url equals 'user', the second
  // part is 'register'
  $arg = arg();
  if ($arg[0] == 'user' &amp;&amp; $arg[1] == 'register') {
    $variables['title'] = 'Test';
  }
}


For the user/register title to be changed in a module:

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'user_register_form') {
  drupal_set_title(t('Test'));
  }
}


Hope that helps.