If you set your mouse sensitivity to 10, it will be set to 9.
I find 10 to be the perfect setting for my mouse as it is the closest feel I get from CS:GO.
I'm guessing this is some type of a weird rounding error?
Mouse sensitivity of 10 is "0.042" in the game engine, 9 being "0.04" and 11 being "0.044".
Having to set them to 9 or 11 works, but 10 never seems to.
The only way is to manually edit your configuration file.