At <http://docs.freebsd.org/cgi/mid.cgi?861r36xzpe.fsf> (short URL for <https://docs.freebsd.org/cgi/getmsg.cgi?fetch=183038+0+current/freebsd-hackers>), 'Next in thread' leads to a list (not a message): <https://docs.freebsd.org/cgi/mid.cgi?db=irt&id=861r36xzpe.fsf@phe.ftfl.ca> Comparable behaviour with another example: <https://docs.freebsd.org/cgi/getmsg.cgi?fetch=832820+0+current/freebsd-hackers> leading to <https://docs.freebsd.org/cgi/mid.cgi?db=irt&id=cd41bda5-6a14-c4e2-3d74-b0c5d52480ec@zohomail.com>
An email message-id is unique (hopefully). Responses to an email are not, there can be zero, one or more than one responses. If there is one response, the script will redirect to the the message. If there is more than one response, you will get a list of responses. In case of an announcement, the list can be pretty long. That's life.