Without knowing particulars, there are really two sets of
headers in each e-mail: the kind that you can "see" -- these
conform to RFC822; and those that you can't (usually) -- these
are RFC821.  The former are usually used to create the latter.
 
It is the latter that is used though for actual transport.
Depending upon the mail systems involved, you might have recipients
that are enumerated in the RFC821 headers, but are NOT in the
RFC822 headers.  Some mail systems allow a user to do that
very easily by use of a BCC: field when they are composing the
e-mail.
 
Obviously other mail transport mechanisms such as LISTSERV
use a similiar vehicle when your subcription distribution
options are SHORT (and others).