Well another reason GL changed up the Vader/Palps dialogue a bit and added "How is that possible?" was to ackknowledge the fact that the Emperor told him he'd killed Padme.
So it was a PT retcon continuity patch.
The reason why it worked w/o that bit of dialogue in the OT (before the prequels existed)was because we as viewers knew no specifics before the prequels.
Depending how Ady will edit RoTsR maybe that line should be kept?