They were simply testing apples to apples. Probably standardized, lowest brightness, light web surfing on wifi. Apple has been noted to offer battery life figure close to what one would get in the real world in the above situation with medium brightness, and not like the typical PC manufacturer where they would say up to 7 hours* (fine print: word processing, screen at lowest setting, wireless turned off).
I'm a power user so I'd probably get closer to 3.5 hours on an 11" Air, whereas my wife would can easily get the advertised 5 hours, if not more. The Hyundai Elantra is an excellent example, it has a small engine that can be very fuel efficient, but it also has the capability of producing 148 HP. Producing 148 HP takes a lot of gas no matter how fuel efficient the vehicle is.
Me and my analogies. 🙂