Bava Batra 163: Empty Lines Invite Forgery

A gap of two lines of between the text and the witnesses' signatures invalidates the document - but does it include the air above and below those lines of text? Also, a gap of even one line is a problem if it's between the court's ratification of the document. But why is a gap of one line acceptable between the text and the signatures - better to sign on the line of the text!