Well, most people, even with heavy usage, get the modern MBA's battery life claims. Remember, the newer models have very power efficient Haswell chips, Mavericks and Yosemite are designed to be very power efficient, they use low-power DDR3 memory and fast, power efficient flash storage. They also have efficient LED displays.

Apple had some estimation issues in the past (when they used the 'industry standard' tests, their new tests tend to be conservative as to what you can expect. The iPad Air is rated for 10 hours, but even in demanding tests most people pulled about 12:30 or so from the battery.