From an E-Mail to the list: 
"[...] XML tooling does not universally provide namespace bindings declared in the containing XML document ([XML 
Namespace] spec is in complete silence whether namespace bindings are in scope for XML *content*), or do not guarantee 
that namespace bindings are up to date to match the prefixes used in the XPath expression. That means that the XPath 
processor has no guarantee that it can resolve all prefixes used in the XPath expression can be resolved to the correct 
namespaces, or even to any namespace at all.

The solution is to make the namespace bindings explicit that have been used to construct the XPath expression."