Excerpt from 4.3 Interpretation of Schema Component Paths
[Definition: A schema component positional predicate is a condition on the relative position of the component in the sequence of matching components from the schema component path step.] A schema component position predicate is true if the position of the target component in the sequence of components selected by the axis and name test is equal to the positive integer given in the predicate. 

The question is about "position of the target component".  It appears to assume the common (though not universal) 1-origin method of counting used e.g. in XPath 1.0 position().  However, if an implementation chose to number sequence of selected components starting with 0 (for the off-by-one and zero-origin crowds), or indeed any arbitrary number other than 1, I find nothing in the words to say such an implementation has transgressed any normative prescription(s).