Why's it slightly noisy? It never resolves to clean when the camera's still, so it doesn't seem to be MC noise for something like ambient occlusion or raytracing/pathtracing, so I assume it's just a 2D effect on top of the image for look?
They put an add-noise filter on top. It's often used to give it a more 'filmic' look and also hides away some uniformity in simple textures. Plain, plan surfaces look very fake, add some noise and suddenly it feels more 'real'.
Plus someone probably decided it would look cool...
