This spate of unsub messages

Mary D. Taffet wrote:
> All you need to do to unsubscribe from this list is examine the full
> headers of a message that you receive from the mailing list.  Right
> there in the full headers is a line called "List-Unsubscribe:".  Just
> click on the link it provides.

Or he could just read the instructions that are posted at the bottom
of nearly every post on the list:

> To subscribe or unsubscribe from this list use the subscription
> manager: <http://lists.xml.org/ob/adm.pl>

I'm curious as to why this doesn't show up on all messages though.