There actually is less gold in it. Apple uses a new alloy that achieves 22 carat but with molecular structure containing more empty space, resulting in a lighter yet stronger alloy. This is not a secret; there’s some good articles out there about it.
No, ctdonath2, it's 18Karat, with a full 75% gold. . . lighter and stronger, but still 75% gold so it retains the 18K rating. Not 22K. See the post above with the photo of the back of the watch.