question about wsdl/rpc

in 
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:wrpc="http://www.w3.org/2004/08/wsdl/rpc" targetNamespace="http://www.w3.org/2004/08/wsdl/rpc" 

why is  attribute QName of  following simple type
- <xs:simpleType name="signatureItemType">
  <xs:union memberTypes="wrpc:directionToken xsd:QName" /> 
  </xs:simpleType>

using  "xsd"  prefix  instead of  "xs"  prefix  ?


  Cordialement,

  Martin FRENAY
  software  -  D7 - surveillances et systemes