This is because Apple is using the iPad Pro 11” case for the iPad Air, and the reason the Air is 10.9” vs 11” is because it has slightly thicker bezels. Not sure why. But good call on Apple as it allows Air users to have the option of using the Magic Keyboard, which I can attest is an amazing accessory on the 12.9.
I think the Air is a compelling product, but honestly, when comparing it to the iPad Pro 11“, both in 256GB, they are $150 apart. For that $150, you get more RAM on the Pro, more powerful CPU (by quite a bit) for multi core, substantially more powerful GPU, 120Hz ProMotion display, 4 speakers, extra camera for the rear with Lidar, and FaceID. If you’re spending $750 on the Air, the extra on the iPad Pro in my opinion is easily worth the extra money, if nothing else for the RAM and display. But again, the new Air is a great product that makes a lot of sense to have in the lineup, as most people probably won’t even know about the differences between the two that I pointed out, and they will see the colors and design similarity and go with the Air.