Neither side is flush on the 4/4S. By making only one side flush on the 5 it's easier to tell which side you grabbed as you pull it out of your pocket/purse/bag so you know to flip it the correct orientation before you ready the device in front of your chest/face.
Every Android phone I've owned is similar in this respect (as are the 3GS and below) but my 4S not so much. This move is for the best.