BING!!! A carb uses changing pressures. The injection needs a consistent pressure in the fuel rail relative to the manifold because it meters fuel flow using time (how long the injector is opened) and that's easier to do accurately when that pressure differential is constant.
Once I got that the mapping is telling the injectors how much time to stay open (richer is longer, duh) they started to make real sense when looking at them. It's fascinating to look at fuel maps, ignition curves and dyno charts together. Then change a value (the shape of the fuel map or Ignition curve) and see the change in the dyno chart, the change in the shape of that curve relative to the others. The first time it clicked was a real Matrix moment....like....I can see the universe man!