Most likely your form was archived. In Guvnor to completely delete a versioned asset is to archive it first, then go to Administration tab -> Archive and permanently delete it there. Currently in Guvnor with its REST api that jBPM Designer extensively uses for communication with Guvnor, archived assets are _still_ parts of the packages (are seen as such) unless you permanently delete them.
Also check out the live-preview form editing in jBPM Designer here: http://surdilovic.wordpress.com/2012/08/06/process-and-task-form-editing-in-jbpm-designer/. It allows you to catch possible errors and preview your form as it will be seen by your users.
Hope this helps.