The difference is, with the iPads and iPhones, a straight doubling the number of pixels across the same physical space had benefits beyond than manufacturing efficiency. For iOS, a pixel-doubled display meant better app compatibility. Pre-existing apps would just run in a 2x mode and developers only had to supply new assets to take advantage of the ‘Retina’ fidelity.
If Apple had chosen any other resolution for the iPhone 4 or iPad 3, developers would have had to redesign their apps from scratch and compatibility mode would have comprised horizontal and vertical letterboxing.
For OS X, unlike iOS, pixel-doubling is not a requirement. Due to the windowed nature of OS X, apps can run on devices with a whole variety of screen resolutions. There may be cost savings for Apple (by using iPad display sheets at a different size), but I can’t imagine that the savings are so significant as to dictate Apple’s course.
Ignoring the above issues, I still think extrapolating the Air’s display from the iPad’s display is a baseless argument. Apple could just as easily use the 15 inch MacBook Pro’s display (which has a DPI of 216), or the 13 inch MacBook Pro’s display (which has a DPI of 226), or something else entirely. Each of those options produce different end results for an ~11inch Air. The point is, you can’t tell.