Because the Air didn't get redesigned.
//...The AIR actually GAINED weight in its recent Rev B. They hit 3.0 pounds and I think they realized if... \\
For what it's worth, the MBA is rather heavy for a sub-notebook. Most of the excess weight comes from the Aluminium housing - dead weight.
That same aluminum housing make the air feel sturdy as hell.I'll take the weight of the aluminum anytime,