When the fisheye distortion is removed or "unwrapped"  to make the image true rectilinear, it stretches out the available image space to stabilize with.  We output at a wider aspect to give users the most F.O.V. possible (crop out as little of the image as possible).  This gives the user the most flexibility to  decide how much more they would like to crop out in the editing program of choice. You can see below in this before and after that the "After" image is not distorted anymore because the checker squares are true and equal sized squares, however the the image is now a wider aspect ratio.