XQuery 1.0 - 3.12.3 and 3.12.5

The cast expression of 3.12.3 and the constructor function of 3.12.5 seem to 
be essentially duplicating the same functionality.

It isn't immediately obvious to me why such duplication should be necessary 
or appropriate, particularly since the cast expression seems sometimes/always 
simply to add the words "cast as" before what would otherwise be a 
constructor function.

Is there a compelling reason for the continuing existence of this seeming 
duplication? Or am I missing something obvious?

Andrew Watt