When Exim is receiving a locally-generated, non-SMTP message on its standard input, the -t option causes the recipients of the message to be obtained from the To:, Cc:, and Bcc: header lines in the message instead of from the command arguments. The addresses are extracted before any rewriting takes place and the Bcc: header line, if present, is then removed.

If the command has any arguments, they specify addresses to which the message is not to be delivered. That is, the argument addresses are removed from the recipients list obtained from the headers. This is compatible with Smail 3 and in accordance with the documented behaviour of several versions of Sendmail, as described in man pages on a number of operating systems (e.g. Solaris 8, IRIX 6.5, HP-UX 11). However, some versions of Sendmail add argument addresses to those obtained from the headers, and the O'Reilly Sendmail book documents it that way. Exim can be made to add argument addresses instead of subtracting them by setting the option extract_addresses_remove_arguments false.

Emphasis added. WTF?