weird behavior: Apache mod rewrite

Hello everyone, this is my first time in this mailing list.
Using: PHP Version 5.2.10-2ubuntu6.4, Apache/2.2.12

I'm redirecting all the request into one file with ( .htaccess ):

RewriteEngine On
RewriteRule .* index.php

My index code is:

<?php  echo $_SERVER["REDIRECT_URL"];  ?>

In the same directory I just have other file besides index.php. And
depends on how this file is named, I'm getting
different values for the same request:

Request: /menu.html

Filename: menu.html
Output: /menu.html
Result: OK!

Filename: menu.html.bak
Output: /menu.html.bak
Result: NOT GOOD!

Filename: menu.html.whatever.other
Output: ERROR 404 (from apache)
Result: NOT GOOD!

Could someone please explain me why I'm getting this weird behavior?

As the request is the same, I expected to display always the same
value. Am I'm wrong??

Thank you.