Excerpt text in WordPress can be customized using the 'Excerpt' meta box on the post edit screen. If you don't customize the excerpt, it is automatically generated. This is the default functionality of WordPress.
My plugin simply allows you to either show or hide the excerpt. Given the settings you describe and the fact that you have set the attribute in the shortcode, the excerpt should definitely not be showing up. However, it is possible that another plugin or a theme is causing a conflict. To test this, you should try switching themes and/or disabling all other plugins. If the problem goes away, then it is a plugin or theme conflict. If you are running a default WordPress theme, my plugin is the only one active and the problem persists, then I would like to follow up with you and try to figure out what could be going wrong.