is that he is turning his head down during the DS so that his face is pointed more towards the ground than out at the horizon. This is the cause for most of the apparent head drop. If I were to post a side view, you'd notice that Scott's forward spine tilt is fairly consistent from address to impact.
Conversely, a non-SA grip will be close to SA at impact but not at address due to the forces of the swing. You have to stand up (i.e. decrease your forward spine angle) during the DS so that you won't drive the club into the ground at impact. This must be done to account for the increased length of the arms/club while transitioning from double to single axis. The timing to do so is difficult and one of the lauded benefits of SA. The stat's I've seen indicate that most tour pro's have a 35* forward spine tilt at address and only about 20* at impact. Pro's like Furyk and Vijay are closer to 5-10* at impact!