Collection-Valued Subexpressions?

I'm reading through the XPath 2.0 Requirements.

    "In XPath 1.0, the use of a collection-valued subexpression can
        introduce an implicit existential quantification or
        choose-first-member operation into the containing
        expression's semantics."

    The problem with XPath 1.0 appears to be that:

        /a[b = 5] 

    Returns all matches while.. 

        /a[b + 1 = 6]

    .. returns only the first match.

    The fix in XPath 2.0 appears to be to return an error for the
    latter if there is more than one match.

    Why is it not possible to return all /a[b + 1 = 6] ?

--
Alan Gutierrez - 
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