minor annoyance with automatic insertion of encoding string

[varro]

I've noticed that when I insert a non-ASCII character into one of my
source files, Emacs automatically inserts an encoding directive like:

# -*- coding: utf-8 -*-

when I save the file.  I assume this has something to do with my
having:

(setq-default buffer-file-coding-system 'utf-8-unix)

in my .emacs file, but I don't understand the reason for this and find
it a bit annoying.  Can anyone enlighten me on why this is happening?

-- 
Will