It's becoming more common over time for used items to be shipped out from Amazon (and other retailers) when a new item was ordered. I've experienced this a lot more myself, over the last few years.
As mentioned, used items are sometimes shipped back and sent out as new. Retailers seem to be getting worse about checking for this, before sending an item out. Hell, I've had quite a number of items shipped in used boxes, that were beat to absolute shit from Amazon over the last year or so. To the point where they were literally scraps, that were tapped together in some cases. I know they're busy, but that's a bit much. One of them was literally made from tape on one side. If it was once or twice, it could be discounted as the carrier doing this, due to shipping damage, but it's happening far too regularly, over many carriers. Boxes were very obviously shipped out used.
Also as mentioned, Amazon has a program now (Amazon Wardrobe) where you're shipped multiples of a clothing item to try out, and you only pay for the ones you keep. All of those (now used) items that were tried out have to go somewhere. They shouldn't be sent out as new after that point, but we know how that goes.