cyrus UIDL - POP double download problem

[Vincent Fox]

So when moving mailboxes from UWash to Cyrus, the obvious solution seemed
to be use imapsync.

I just did a test-run of it moving my own mailbox from UW to Cyrus.  It
looks like the UIDL changes in the process, so any POP client is going to
do double-downloads of INBOX contents.

Is there any way to preserve the UIDL information? I took a look at RFC1939
and it leaves the implementation of how this is derived to the software. I
gather it's typically a hashed valued of the message contents, so I would
have thought a message would have same hashed value on 2 different server.
Apparently not. How does Cyrus derive the UIDL?