16mm film ultra wide lenses

So, I know because of field of view on 16mm, that a lens designed for 35mm film, say a 50mm, would read like it's a 100mm when put on a 16mm camera (I may have worded that poorly, but I think I grasp the concept)

My question is, why are some 16mm lenses so damn wide? I was consistently seeing 6mm and even a 4.5mm and 3mm. Again, I know these "appear" twice as long on 16mm, but there's nothing equivalent to those in 35mm cinematography unless you're using insane fisheye lenses. Am I misunderstanding the conversion, for lack of a better word. Is it more than 2x?