One possibility is that the pgp-users mailing list is "shared" with
the comp.security.pgp.discuss newsgroup (by that I mean: whenever
someone posts an article to the mailing list, THAT SAME ARTICLE is
also posted [by the mailing list server] to the newsgroup).
Since you are able to 'expire' your mailing lists, I assume you have
set them up as "pseudo-newsgroups". As far as I know, Yarn stores
only a single copy of any "news" article (identified by its unique
message-ID), no matter how many "groups" that article appears in.
If the article disappears from the c.s.p.d news group when you expire
the mailing list, I would assume the article you would have seen if
you had looked at c.s.p.d would have been _identical_ to the article
you already viewed in the mailing list. If the article does _not_
disappear from c.s.p.d even if you expire every article on the mailing
list, well, that article was posted ONLY to c.s.p.d (i.e., you did NOT
get to see that same article when you viewed the mailing list).
mikus