I believe the reason for not having this column was that it's already obvious that all the documents are about this agenda.
IIRC agendas were added fairly recently to the site (after the documents table was added) so I think the actual reason is simply that I haven't gotten around to syncing everything up on the site. I appreciate you bringing this up!
EDIT: oops, I just realized this was the agenda page, not the organization page, so my reasoning above is totally wrong. I think you're right about the actual reason.