SJSXP (StAX implementation) abode by the StAX spec and returned null when there is no version information. The issue is with the jaxp in jdk. When the StAX implementation was integrated into jaxp and then jdk, the common Apache parser for DOM and SAX was also used for StAX. The common parser does set default version to 1.0. The getXmlVersion for dom Document was defined like this:
"... If there is no declaration and if this document supports the "XML" feature, the value is 1.0 ..."