Table's key pointing to its own table?

Question

class diagram http://img223.imageshack.us/img223/8558/classdiagram.png

In this image, why is the EmployeeID pointing to it's own table while the order id is not pointing to the order's table. The screenshot is a picture of the northwind database.

Answer 1

Because employees [EmployeeID] report to other Employees [ReportsTo]. An order belonging to another order hurts my brain.