"silent redirects" using .htaccess

Most .htaccess mod_rewrite rules I've written have "silently"
redirected to the new URL. This one, however, does not:

RewriteEngine on
RedirectMatch ^/(?!new) /new/$1

Instead of being "silently" redirected, I'm redirected via a Location
header, and I don't want to be.

Any ideas as to why I am?