Field name not same in XML

Is there any reason for which WebService returns xml attribute name as _x0031_00GContains=some value and not 100GContains=some value ? (Column name is 100GContains)

Yes. XML element/attributes cannot start with numbers.

From http://www.w3schools.com/xml/xml_elements.asp:

XML Naming Rules

XML elements must follow these naming rules:

  • Names can contain letters, numbers, and other characters
  • Names cannot start with a number or punctuation character
  • Names cannot start with the letters xml (or XML, or Xml, etc)
  • Names cannot contain spaces
  • Any name can be used, no words are reserved.