Why does setting event retention starttime generate a new XUID?

I am trying the event based Retention Management using XAM apis.  Here are the steps:

1) During creation of the XSet, I set the the following base and event retention fields:

xSet.createRetention(true, "event");

xSet.setRetentionEnabledFlag("event", false, true);

xSet.setRetentionDuration("event", false, 1000);

xSet.setBaseRetention(false,0);

2) After creating, I set the event starttime:

     xSet.setRetentionStarttime("event",false);

When I commit after setting the event Start time-- I get a different XUID.

3) When I delete using the 2nd XUID  - It is successful.  However, it takes a while before the XSET is deleted and inaccessible.

For a few minutes, i am able to get it and iterate over its fields.  Why is that?

4) When I try to delete using the original XUID, its unsuccessful because its considered under retention (looking at the fields the event starttime is not set on this XSET.

I want to set event starttime  on the same XUID (i.e., non-binding).

Attaching the code below:

StoreContentWithRetention.java -- creates the XSET

ManageRetention.java - sets the event StartTime

DeleteContent.java - deletes

thanks