"Greg Esres" wrote in message
...

The only possible thing that I could think of is that if ATC were
waiting to assign that altitude to another aircraft. However, I would
think that ATC would not assign a discretionary descent if that were
the case.


Exactly. If the controller needed ten thousand for another aircraft, he
might issue "descend now to niner thousand, then descend at pilot's
discretion maintain four thousand" instead of "descend at pilot's
discretion,
maintain four thousand".