Additional questions on non-deterministic content model

Hallo NG,
 
I know, that I have understood the principles of a deterministic content
model.
However, I would be lucky, if someone could explain the following questions,
thank you.
 
- In the XML-Spec there is a non-normative (Appendix E) and a normative
reference concerning
  this problem:
       For compatibility, it is an error if the content model allows an
element to match more 
       than one occurrence of an element type in the content
model.<?xml:namespace prefix = o ns =
"urn:schemas-microsoft-com:office:office" />
  Is it correct, that a parser is not required to throw an error, although
this reference is normative?
 
- Why does the majority of XML Schema parsers throw an error, if a
content-model is non-deterministic,
  while DTD parsers don´t ?
 
Thank you
 
Rainer