[exim] Wrong documentation for server_set_id?

Hello,

I have an authenticator that uses the full email address as username, so
it results in:

     $auth1 = roland@???
     $auth2 = password

In the authenticator, I have:

     server_set_id = $auth1

The documentation[1] says that this variable is only used for logging
and for user-defined things. But somehow, it is also embedded in
outgoing mails, so that it looks like this:

Return-Path: <"roland@???>
...
Received: from c183123.adsl.hansenet.de ([213.39.183.123])
    by p15097294.pureserver.info with esmtpa (Exim 4.67)
    (envelope-from <"roland@???>)
    id 1IcGSX-0008TP-Fi
    for roland.illig@???; Mon, 01 Oct 2007 10:16:57 +0200
Message-ID: <4700ACF9.6070207@???>
Date: Mon, 01 Oct 2007 10:16:57 +0200
From: Roland Illig <roland@???>
...
Sender: "roland@???

Is the documentation wrong, or did I understand it wrongly?

Roland

[1] http://www.exim.org/exim-html-current/doc/html/spec_html/ch33.html