BuddyPress.org

I would be able to specify expires access for the BuddyPress contents. It’s OK for WordPress posts, as you can see :
http://www.paul-emploi.biz/actu/anonymous-attaque-emploi-en-france/15945/

HTTP/1.1 200 OK
Date: Tue, 08 May 2012 10:51:22 GMT
Server: Apache
X-Pingback: http://www.paul-emploi.biz/xmlrpc.php
Set-Cookie: bp-message=deleted; expires=Mon, 09-May-2011 10:51:22 GMT; path=/
Set-Cookie: bp-message-type=deleted; expires=Mon, 09-May-2011 10:51:22 GMT; path=/
Link: ; rel=shortlink
Cache-Control: max-age=604800, public
Expires: Tue, 15 May 2012 10:51:22 GMT
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 15175
Keep-Alive: timeout=2, max=150
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8

But it doesn’t work for forum topics and activities :

http://www.paul-emploi.biz/groups/techniques-de-recherche-demploi/forum/topic/le-cv-video-ca-marche/

HTTP/1.1 200 OK
Date: Tue, 08 May 2012 11:53:31 GMT
Server: Apache
X-Pingback: http://www.paul-emploi.biz/xmlrpc.php
Expires: Wed, 11 Jan 1984 05:00:00 GMT
Last-Modified: Tue, 08 May 2012 11:53:32 GMT
Cache-Control: no-cache, must-revalidate, max-age=0, public
Pragma: no-cache
Set-Cookie: bp-message=deleted; expires=Mon, 09-May-2011 11:53:31 GMT; path=/
Set-Cookie: bp-message-type=deleted; expires=Mon, 09-May-2011 11:53:31 GMT; path=/
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 11062
Keep-Alive: timeout=2, max=150
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8

http://www.paul-emploi.biz/members/phgras/activity/604/

HTTP/1.1 200 OK
Date: Tue, 08 May 2012 10:49:16 GMT
Server: Apache
X-Pingback: http://www.paul-emploi.biz/xmlrpc.php
Expires: Wed, 11 Jan 1984 05:00:00 GMT
Last-Modified: Tue, 08 May 2012 10:49:17 GMT
Cache-Control: no-cache, must-revalidate, max-age=0, public
Pragma: no-cache
Set-Cookie: bp-message=deleted; expires=Mon, 09-May-2011 10:49:16 GMT; path=/
Set-Cookie: bp-message-type=deleted; expires=Mon, 09-May-2011 10:49:16 GMT; path=/
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 10598
Keep-Alive: timeout=2, max=150
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8

Is it a difference between WordPress posts and BuddyPress contents for ExpiresByType text/xml access ?