What's the different between iPad Retina simulator and iPad Air simulator in Xcode 6?

Question

An iPad Air already has a Retina screen, so why has Apple deliberately designed two kinds of simulator?

Answer 1

The simulator does not include the iPad mini. The difference between the Retina and the Air is the 64 bit processor.

iPad Air = Retina x64
iPad Retina = Retina x32
iPad 2 = Non-Retina x32

To test the iPad mini Apple recommends testing on a non-retina device (aka iPad 2):

From the "Apple iOS Simulator Guide":

Although an iPad mini isn’t an option for a simulated device in Simulator, you can still test apps for the iPad mini in the simulator. To do this, run your app on the simulated iPad without Retina display.

Edit:
You can also test iPad mini on iPad Retina Simulator.
See updated Apple "iOS Simulator User Guide":

Testing for the iPad mini
To test apps for the iPad mini in the simulator, run your app on a simulated iPad with the corresponding type of display, either Retina or non-Retina depending on the iPad mini model.