I was reading the documentation, and i didn't quite understand that part in the Swimlane section (http://docs.jboss.com/jbpm/v3/userguide/taskmanagement.html)
Important to know is that all assignments that are done on the task instances in a given swimlane will propagate to the swimlane instance. This behaviour is implemented as the default because the person that takes a task to fulfilling a certain process role will have the knowledge of that perticular process. So all subsequent assignements of task instances to that swimlane are done automatically to that user.
That part simply means that a swimlane is assigned an actor ID only once, when the first task using that swimlane is instantiated. Subsequent task instances use the same actor ID.