User Register page is different if there is a destination parameter in the url
The drupal_get_form() returns a form array for rendering. In your case drupal_get_form('user_register_form'). You can use drupal_render() with the result to render the html output.

If you need to override the form, use hook_form_alter() and change the $form_state['redirect'][] = 'your/redirect/path'. Also try removing the destination param with unset($_GET['destination']).

Here are some references:


Form Redirect not working if &#39;destination&#39; is in URL
https://stackoverflow.com/questions/1175687/redirect-after-registration-in-drupal
http://www.brianvuyk.com/story-type/changing-redirect-value-drupal-node-form-d6