Action attribute in forms is not a url. Why?
If you look again, I would hazard a guess that you are using pathauto, and that the default pattern for Webform paths is [node:title]. If you set it to form/[node:title] then the action attribute would look more like a path. eg. My Form would have an action attribute of action="/form/my-form".

In Drupal everything goes through the Drupal bootstrap. There are no calls to cgi-bin or the equivalent. Rendering a form or processing a form are done by calling the same url, which tells Drupal where to look for the form. What Drupal does depends upon the op associated with the call, and the rendering, validation, processing and storage are all defined by the modules underlying the form.