Alot of this may have to do with the design of golf clubs themselves. Golf clubs are designed with a forward lean in the shaft when soled properly. A wedge has more forward lean than a 5 iron which has more lean than your driver. So if you kept your impact fix hand position constant for those 3 clubs, you would see why the need to move your ball position. Chuck Evans demonstrates this concept on his "How to build your golf swing" DVD.