(quoting from XSLT Programmer’s Reference, Michael Kay, Wrox Press, 2000 page 387):

//section/para[1] returns all para elements that are the first para child of a section parent.
(//section/para)[1]returns the first element in the document that is a para child of a section parent.

XIA treats the first case as the second, and finds the second case syntactically incorrect.