The edge of the iPad is aligned exactly to the edge of the magazine.
This framing suggests that the iPad is to scale and isn't scaled down (as it might be assumed to be if it were centered). All of the blank space is also supposed to give you a sense of how small it is relative to the magazine.
When I first turned over my copy of the issue, I was holding the magazine by the corner exactly as one would hold a portrait iPad mini. In addition to showing the actual size, I think having it aligned with the edge of the magazine is meant to give the impression of holding an iPad.
Anecdotally, I thought the effect was neat, and spent a few more seconds than I otherwise would have on it.
This framing suggests that the iPad is to scale and isn't scaled down (as it might be assumed to be if it were centered). All of the blank space is also supposed to give you a sense of how small it is relative to the magazine.