SetEnv variables renamed by mod_rewrite

Hello.

[Context: Apache 2.2.2, PHP 5 / CGI.]
Let's say I set an environment variable in an .htaccess file:

SetEnv Test Okay

Now I request a PHP-page which is redirected internally by mod_rewrite.
I find my Test variable renamed to REDIRECT_Test.

1) Why does mod_rewrite do this?
2) How can I prevent this?

Greetings,
Thomas