The mode line in both instances specifies the file permissions of the various files created for each service. In the case of 0660 your specifying that the file can be read and written to by the owner and group (both postfix in the lmtp case) and no access for local users that are not a) postfix and b) not members of the postfix group, essentially everyone else.
In the case of your configuration LMTP is acting as a service endpoint for dovecot and is being created as a UNIX IPC Socket in the /var/spool/postfix/private directory owned by postfix, group id of postfix and
srw-rw---- as the permissions.
The auth service is creating two listeners, both UNIX IPC Sockets; the first in /var/spool/postfix/private named auth and the second is a special socket called auth-userdb used to gather the uid, gid and home directory of the recipient. Both of these files will have the permissions
srw-rw-rw-, the latter being owned by vmail. The process that runs the local delivery agent will be the owner of the auth socket.
Hopefully this helps, I've not used LMTP much nor Dovecot for that matter but this is what I could gather from the documentation on their wiki. The permissions and sockets are fairly standard.