about auto-mode-alist

[step]

  hi,i use gnu emacs23 ,when i set the auto-mode-alist, it seems don't
work.
  the elisp i add in the my .emacs file follow:
   (setq auto-mode-alist
  (append '(("\\.css\\'"  . css-mode)
            ("\\.c\\'"    . c-mode)
            ("\\.cc\\'"   . c++-mode)
            ("\\.cpp\\'"  . c++-mode)
            ("\\.cxx\\'"  . c++-mode)
            ("\\.hpp\\'"  . c++-mode)
            ("\\.e\\'"    . eiffel-mode)
            ("\\.hxx\\'"  . c++-mode)
            ("\\.h\\'"    . c-mode)
            ("\\.hh\\'"   . c++-mode)
            ("\\.idl\\'"  . c++-mode)
            ("\\.ipp\\'"  . c++-mode)
            ("\\.java\\'" . java-mode)
            ("\\.lua\\'"   . lua-mode)
            ("\\.pl\\'"   . perl-mode)
            ("\\.pm\\'"   . perl-mode)
            ("\\.rb\\'"   . ruby-mode)
            ("\\.rbw\\'"  . ruby-mode)
            ("\\.t2t\\'"  . t2t-mode)
            ("\\.txt\\'"  . text-mode)
            ("\\.py\\'"   . python-mode)
            ("\\.tex\\'"  . LaTeX-mode)
            ("\\.\\(xml\\|xsl\\|rng\\|xhtml\\)\\'" . nxml-mode)
            )
      auto-mode-alist))
   when i use the C-x C-f create a new .xml file ,it can auto load to
nxml-mode ,but if i open a exist .xml file ,it auto load to sgml-mode
which is the default mode emacs set in the file.el .
   is there somebody konw why? thanks!