a bug or story?

I was walking around the nether and found this, is it kind of a world bug? because stone and obsidian in the nether, it's impossible to have, because: 1: the stone is made when the lava falls on top of the water, and the obsidian is made when the water falls on top of the lava ... que: nether was a beautiful overwold that ended up destroying itself and became the dark nether?