Issue is on this page: https://api.drupal.org/api/drupal/modules!image!image.module/function/image_style_url/7
This is no longuer true since drupal 7.20: Requesting the URL will cause the image to be created.
From: https://drupal.org/drupal-7.20-release-notes
In addition, any code which programmatically generates a link to an image derivative without using the standard image_style_url() API function will no longer work correctly if the image does not already exist in the file system, since the necessary token will not be present in the URL.