Entourage is deleting the identifiers at sync engine and adding its own ids

[BB]

Hi,
I am writing a sync tool that sync entourage with my backend server.
Now created some event at the server and now using my tool I have synced it
with entourage now. if I am updating this event from the entourage and
syncing it with my backend server using my sync tool then its does not
getting updated.
the reason why its not getting updated is that the entourage is deleting the
identifier I had provided to the sync engine. and adding its own identifiers
at the syync engine. so when I do a sync its not able to recognise that some
old event has been updated.
Can some one suggest me why entourage is adding its own identifiers while
the same thing is working fine with iCal.
iCal is using the same identifiers I have provided while pushing the events
the sync engine in the form of sync records.

Thanks,
Bala