Here the ambiguity is between a statement about a statement about a document, and a statement about a document. Another example is an element which qualifies another apparently element. When information is assembled in a set of independently thrown in records often ambiguities can arise because of the lack of logic. These things can work when one program handles all the records, but when you start mixing records you get trouble. Another symptom of a specification where the actual semantics may not be as obvious as as first sight is ordering.
Independent assertions are always valid taken individually or in any order.
That isn't what the spec said, but it did mention that the rules were processed in order until one applied. Represented logically, in fact there is a large nested conditional. In email headers the logic of this has to be written into the fine print of the specification. There is something fundamentally different between giving a machine a knowledge tree, and giving a person a document. A document for a person is generally serialized so that, when read serially by a human being, the result will be to build up a graph of associations in that person's head.