I can confirm this obversvation. Once when I was mage and solo farmed wind dragon skulls for my melee build, I got a skull+ in ToG for a D-rank after quitting with only one drake killed. Several times the box was completely empty (nothing in it), but once in a sudden I got the skull as the only reward.
As for S-rank I do not think it gives you the better rewards but more pieces in the box.
Last edited by Riff-Raff; 10-08-2010 at 12:41 PM.