What you've missed is that it didn't get lighter (really), it just got thinner. The reason it got thinner is that they eliminated the air gap in the screen, which didn't save much weight (some construction around the air gap, I guess).
The smaller battery presumably speaks to having to shift things around, and possibly some weight went in to reinforcing the cassis which made up the difference.
The other side of this is that if they had kept the thickness of the original air, it would have had to be heavier by as much as 25%. Sure, it would have been great for battery life, but my iPad 3 is testament to the fact that dropping the weight to the air was necessary and that they had to keep the weight down in the Air 2.
The smaller battery presumably speaks to having to shift things around, and possibly some weight went in to reinforcing the cassis which made up the difference.
The other side of this is that if they had kept the thickness of the original air, it would have had to be heavier by as much as 25%. Sure, it would have been great for battery life, but my iPad 3 is testament to the fact that dropping the weight to the air was necessary and that they had to keep the weight down in the Air 2.