I suppose you could say I have/had the same problem.
My battery life on my 11" MBA is similar to your situation. I fully expected that the technician would find nothing wrong, as battery life varies. A perfectly functionaing battery can yeild quite different run-time expectancy, depending on the nature of the use of the device.
Some of the reposnders here have been using their experience with a 13" MBA, which has a much longer run-time, due to its higher battery capacity versus load ratio. "The 11-inch MacBook Air has a battery that lasts up to 5 hours, and the 13-inch model lasts up to 7 hours", according to Apple's battery testing protocol. The operative words in that description is "up to".
Maybe I am coming on a bit strong here, but this is what I have found after owning 3 MPP's and one MBA, is that battery charge longevity is wholly dependednt on how much load you put on the components by the nature of the use of that computer. By saying your manner of use is "Safari only", doesn't tell much. Safari itself doesn't use much in the way of resources, but the plug-ins and support applications it may use can easily add to the overall amperage draw by a factor of many times Safari alone. That in itself can mean the diffrence between 5 hours, and 3. The manner in which you use Safari, the way users here use Safari,and the manner in which Apple tests Safari for baatery longevity, is probably qite different. I doubt Apple even tests Safari with Flash, seeing how they dont't even include it anymore.
But hey, if Apple is willing to swap out the unit to give you peace of mind, and if it please you, then good for you! I hope that I am wrong about your situation, and all goes well.
BTW, I am greatly pleased with all my Macs, especially my MBA, which gets 5 hours battery when I use it lightly, and 3 hours quite often.