Put your DPI on something that feels comfortable in Windows (anything >=800 should be ok, go lower and you lose resolution afaik). Leave it at that and from now on ONLY adjust the sensitivity ingame. Then go the controls menu ingame and reset to defaults in order to start from scratch.
Now, you're understanding the entire sensitivity system wrong i believe.
- "Look Sensitivity" (Hip sense) is the main sensitivity and determines how your mouse movement translates to ingame movement.
- "ADS Sensitivity" is a multiplier of the Look Sens (it says it right there in the menu!).
- kinda the same goes for the scope sensitivities, menu says they stack with ADS, whatever that means. As a rule of thumb though: Numbers <1 will reduce sens, >1 will increase it.
ADS Sens = Look Sens value * ADS Sens value
0.43 * 0.43 = 0.185
So it's no wonder ADS goes down, because you multiply a value with another value below 1, which further decreases sensitivity. (For the difference of 400 and 1000 DPI.. either the game compensates somehow or 400 is so low already that you don't notice the difference between hip and ads sens. Unsure.)
Hope that helps!