Tom - I've often viewed that design strategy of benching bunkers into a hillside as a sort of leitmotif of Flynn's style. You see it everywhere - numerous instances at Shinnecock (10,14, etc.), HVGC (18) and numerous other examples.
It's almost as if Flynn was trying a little trompe l'oeil making the transition section appear as part of the playable property to fool the golfer into thinking the elevation change isn't as severe. But then again, perhaps it just aesthetic.