Here is the thing... When you are talking about a device like this, the only way to bend it is either it came that way and nobody noticed, or it took a lot of pressure to bend it.
That's the downside of making phones thinner and putting aesthetics over structural integrity. The iPhone 6 suffered from it, as does one of the new Nexus phones.
It's easy to see how it can happen to a phone, because we all toss them in a pocket and sit down or something... But a tablet?
If it wasn't you, then someone else either sat on it or stepped on it, maybe without realizing it was in the bag, or under a magazine or something... That's the only way this could happen, and the person at the Apple store is likely gonna know this. If it wasn't a bad unit that slipped through QC, then someone had to have accidentally done this to your iPad. Sucks.