UniqueProperty, UnambiguousProperty

Why is UniqueProperty a subclass of

   rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property

whereas UnambiguousProperty is a subclass of

   #ObjectProperty?

This seems especially odd in view of this statement (from the language
reference doc): Note that the inverse property of a UniqueProperty is
always an UnambigousProperty and vice versa.

I sort of thought a design decision had been made that each DAML
property should be either Object or Datatype, and that this distinction
was supposed to be made clear in its declaration (someone please correct
me if I'm wrong).  But, if that's right, UniqueProperty allows one to
get around that design decision.

- David