There are couple of things going on here
1 Apple increased the battery capacity
2 Apple chose the lower clocked version
So these might be contributing to the battery life increase - it may NOT be just haswel battery life improvement.
+1 Haswell only saves power in idle and some GPU operations. I would venture a guess that the lower clock and larger battery contribute about 80% of the increase in capacity.
It would be nice, if Apple let the user choose between speed and battery life instead of decreasing the clock speed by default.
Apple did not choose lower clocked CPUs. Haswell ULV runs at lower base clocks, but the max turbo frequencies remain the same or increase slightly over Ivy Bridge. As long as the system can manage to dissipate the thermal load, Haswell will crank away at max turbo without throttling until its work is done and it can get back to sleep.
The 2012 MBA's came in the following flavors:
11-inch: Core i5 3317U, max turbo 2.6 GHz
13-inch: Core i5 3427U, max turbo 2.8 GHz
11 and 13-inch: Core i7 3667U, max turbo 3.2 GHz
The 2013 MBA's are configured thusly:
11 and 13-inch: Core i5 4250U, max turbo 2.6 GHz
11 and 13-inch: Core i7 4650U, max turbo 3.3 GHz
The high end spec actually has an effectively higher clocked CPU. Also, the base and turbo frequencies for HD 5000 graphics are lower than they were for HD 4000, but performance is up across the board due to architectural changes. The only regression was the less expensive 13-inch model, where Apple had the option of going with an i5 4350U with a base clock of 1.4 GHz and max turbo of 2.9 GHz. I'm sure Apple had their reasons, especially seeing as Intel lists the i5 4250U and i5 4350U at the same suggested price.
The battery life improvement is almost entirely due to Haswell. Besides the new battery chemistry and LPDDR3 RAM, there is no other new magic at play here. The effect of the battery chemistry is a very straightforward calculation and results in an increase in available energy of less than 10%, yet the advertised run-time is increased by more than 70%.
