emacs refuses to save file in the same encoding it was read

[Sam Steingold <sds@gnu.org>]

GNU Emacs 21.3.50.1 (i386-msvc-nt5.0.2195)
 of 2003-10-08 on WINSTEINGOLDLAP
--with-msvc (12.00)

I have a file which starts with this line:
;;; -*- coding: utf-8-unix -*-

it is opened and displayed by emacs correctly.
however, when I try to modify and save it, I get the following question:

Selected encoding mule-utf-8-unix disagrees with iso-2022-7bit specified
by file contents.  Really save (else edit coding cookies and try again)?
(y or n)

when I answer affirmatively, the file is saved, but a line
;; -*- coding: iso-2022-7bit; -*-
is added in front of my cookie.

what is going on?

My understanding is that since everything can be encoded with utf-8,
there is no reason for Emacs to suggest a different encoding.
I seem to remember that there are some identical characters - from the
unicode POV - which Emacs represents differently.
First, I thought this bug has been fixed already.
Second, I thought it was irrelevant here anyway: the file is read in
utf-8!

It appears that buffer-file-coding-system is somehow set to
iso-2022-7bit; I sets default value to utf-8.

-- 
Sam Steingold (http://www.podval.org/~sds) running w2k
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main(a){printf(a,34,a="main(a){printf(a,34,a=%c%s%c,34);}",34);}