It's not a sensitivity on it's own, it's a multiplier for the SoldierSensitivity (hipfire) and the various aims via SoldierZoomSensitivities. So you can't make the global sensitivity match anything since it's worthless without the additional values. Setting it to 0.1, 0.5 or 1 basically means nothing until you also have the SoldierSensitivity/SoldierZoomSensitivities values.
The "All" aim does not lock it to 1, it uses 1 as a placeholder value (because it's default by the game) to make calculations easier. You can use any value you'd like within the range of the sensitivity, but using values other than 1 is seldom needed.