What design decisions were made on the way from Apocalypse World to Dungeon World? 

(I'm really interested but a bit mystified because there's so much to talk about now.)

I know what to do when a player joins the game.  Re-do the Hx rules. 

What expectation should I set when a player can't make a session but their character is materially involved in the evolving situation?

What's the balance between:
"Make Apocalypse World seem real."
"look at NPCs through the crosshairs" and
"be a fan of the players' characters"

(I'm not blasé about spotty attendance; I try to avoid having spotty players take anything more than guest roles in my games, but once in a while something's going to give)

So this seems like a pretty natural move - something comes up in play / an NPC mentions he's from the Salt Flats out east, and the players ask "What do I know about the Salt Flats?"

Obviously there are guidelines - the MC barfs forth, makes the world seem real, announces future badness but misdirects, whatever. 

But I was curious why it wasn't an actual move, since it seems like you could make a chart like

When a player character uses their noodle, roll +sharp.
On a 10+, you know one, on 7-9, both.
- You've heard something more than its name and basic description.
- What you've heard is from a reliable source.
On a miss, the GM tells you something that you believe is a hard fact. 

I assume it has to do with player autonomy over their characters' brains, or player/character disconnect on the miss, but it also seems like it would pull play along so I was wondering why the downside outweighed the upside. 

How important is the existence of five stats + Hx, instead of say, 4 or 6 stats, with or without Hx?

Is there a ratio of

Stat:Moves built for a stat

that applies? 

i.e. if I wanted to have 4 stats an no Hx, should I also expect to have fewer moves?