Camera Pseudo-FOV: Why?

Nine days ago, I posted this topic on Scripting Support. The issue was simple, orientating your camera up or down causes a strange warping effect, similar to increasing the FOV of a camera. The solution to this was that I had forgotten to use Unit, which created awkward CFrames with the UpVector magnitude smaller than 1. However my only question still remains: why does this exist? Is this a feature or a bug?