question on key localizaton

=?ks_c_5601-1987?B?s6rH9sDN?= <[email protected]>
Newsgroups gmane.ietf.snmpv3
Message-ID <[email protected]> 
According to key localization algorithm ( specified in RFC2574 section 2.6 ), 
authentication keys ( or privacy keys ) on every authoritative engines are all different for a single user.
Why is this necessary ?
Should an engine know the others' keys which are all different for a single user for communication?

Am I misunderstanding something?
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