Because the iPad Air 2 has an aperture of f/2.4, and the iPhone has an aperture of f/2.2. This makes the iPhone camera brighter and ideal for lower light conditions, but it also changes the focal length to the image sensor. The image sensor itself on the iPad Air 2 is also inferior, as it is smaller with a pixel pitch of 1.12 microns, and the iPhone 6 has a pixel pitch of 1.5 microns. This makes the iPhone have a cleaner sensor with less noise and I believe higher dynamic range and sharpness. Therefore you have different optical math going on between the relationship of the maximum aperture, and the size of the image sensor, which on the iPhone 6 necessitates a further distance to achieve focus. It's the same reason not all dSLR lenses are the same length, although telephoto capabilities do come into play and kind of ruin that comparison to a certain extent. For example, I shot the Mizzou football game against UCF and Indiana a few weeks ago. My Canon 400mm f/4L is much smaller than my coworker's rented Canon 400mm f/2.8L. But his backgrounds were a lot creamier than mine and I was jealous!