I've seen a lot of confusion about League changing the Windows sensitivity or not being the same speed as on the desktop and people reporting it as a bug.
Here is a quick explanation of how in-game sensitivity works and how to match it with your Windows speed.
Even though Windows only has 11 pointer speed notches in the control panel, it has 20 sensitivity steps available. These can be accessed through the registry.
* Windows + R > regedit > HKEY_CURRENT_USER\Control Panel\Mouse > MouseSensitivity
League of Legends doesn't have its own sensitivity control. The game uses one of the 20 Windows sensitivity steps while in-game and reverts it afterwards.
* League of Legends.exe starts > reads the registry key MouseSensitivity > saves it in the game.cfg as SystemMouseSpeed > changes the registry key to the GameMouseSpeed value
* League of Legends.exe closes > reverts the registry key to the saved value of SystemMouseSpeed > changes the value in the game.cfg back to SystemMouseSpeed=0
Every in-game mouse speed step corresponds to one Windows sensitivity step.
If you want to match your Windows and in-game sensitivity there are three ways of doing it:
1. Set them to the same value as shown in the chart above in-game and in the control panel
2. If you use a in-game value that's not available in the control panel, set the registry key MouseSensitivity to the value of your GameMouseSpeed in the game.cfg
3. Set you Windows pointer speed to 6/11 and you in-game mouse speed to 50 and change your mouse CPI to the actual CPI that you've been using (CPI x Multiplier = actual CPI)
CPI 1600 and Mouse Speed 40
1600 x 0.75 = 1200 actual CPI
This is the best way if you can adjust your mouse CPI, because you don't lose pixel precision from not using 6/11 in Windows.
If your Windows sensitivity is < 6/11 Windows will drop mouse counts.
If your Windows sensitivity is > 6/11 your cursor will skip pixels.
The result is uneven cursor movement, especially when not using 4/11 or 8/11.