"Despite using relatively power efficient hardware and being paired with a 30Wh internal battery, the Chromebook 11 barely lasted 5.4 hours in our web browsing battery life test. Local video playback was even worse at 4.8 hours."
The "web browsing battery life test" in this article is the same one as in here: http://www.anandtech.com/show/7117/haswell-ult-investigation. Note how the iPad 4 gets 9.48 hours on both tests. On that test, the 11" MBA gets over 11 hours, despite having less than a 25% larger battery and using Core i5 processors. The 13" MBA gets over 14 hours, though it has an 80% larger battery (but has to drive a 30% larger screen).
This suggests that Samsung's Exynos 5 is extremely power-hungry or Chrome OS's power optimization is extremely bad.
As an aside, Apple/Intel's phenomenal battery life performance with the latest MBA signals to me the beginning of the end for ARM. If we're getting to the point where a current-generation Haswell laptop CPU is putting up light workload power draw comparable to a flagship A15 tablet chip, that's not a positive thing for ARM. Heck, even if you just compare the iPad 4 with the MBA 11", the writing seems to be on the wall. The iPad has a substantially bigger battery than the MBA, while driving a smaller (albeit more pixel-dense and power hungry) screen. And yet the MBA totally holds its own on the battery life test.