The problem is that the underlying thread (in this case, the object xmlDoc) uses UTF-8, which is the default encoding in .NET.
From the MSDN documentation The encoding property in XmlWriterSettings:
XmlWriter, , . XmlWriter TextWriter, Encoding TextWriter. , Unicode (UTF-16) XmlWriter, StreamWriter ( TextWriter) , UTF-8, UTF-8.
, XmlWriter , 1. , XmlDocument .