I can't speak how for graue, but I can offer my experience since I type almost the same way as graue does, except that I do use my ring fingers occasionally (mostly right ring finger, for stuff like "L") and my pinky for stuff like right shift and enter.

How and why? I started messing around with programming when I was a kid and I never learned typing or trained for it formally. When I started, I was just pecking with my index fingers, but since my mom and dad used a typewriter for their stuff, I was fully aware that pecking was suboptimal. They never offered any practical help with that, probably because they knew that I couldn't care less about how I type, as long as I could crank out my code. Long story short: I've developed my own technique, spontaneously. I'm aware that it's probably suboptimal, but it's good enough for me and I can't be bothered to retrain when there's so much more interesting stuff to do ;)

I have an interesting way of typing based on what I did as a kid too. Due to the large amounts of games I played, my left hand stays firmly over the wasd keys and my right hand moves a lot more than my left. I tend to type really loud and mostly use my index fingers, but also switch to using various other fingers when I'm typing quickly. I know its sub optimal, but I hit 80 wpm pretty consistently.