Slightly higher clock speeds and more thermal headroom explain the differences.
iPad mini 2 review
A7: How Apple's custom 64-bit silicon embarrassed the industry
Apple iPad Air: The TechSpot Review > Wrap Up: Is This Apple's New
Apple iPad Air 2 & iPad Mini 3: Evolutionary, not revolutionary
Apple announces iPad Air, iPad mini with Retina display
iPad Air graphics rated up to 70% faster than iPad 4
Apple's A8X powers iPad Air 2 graphics faster than Google's Nexus
iPad Air 2020 vs. iPad Pro 2021 Buyer's Guide - MacRumors
October, 2013
iPad Air's A7 chip is identical to the iPhone's, just faster