IBM-DM-017: Problems with attribute node, string-value accessor

Data Model Section 6.3.2 (Attribute Nodes--Accessors): Why doesn't the 
dm:string-value accessor simply return the string-value of the attribute, 
since it is a node property? If the string-value accessor returns 
something other than the string-value property, then why does the node 
have a string-value property? Why are we talking about things like 
"original lexical representation", "normalized value", "explicit 
timezone", and "tuple representation" which are not stored in any property 
of an attribute node?