Mouse acceleration is for "wide" movements. It enables you for to go through the entire screen without large mouse movements (a kind of warp mode)
Mouse sensitivity is the exact opposite. For your very small movements, you'll have smaller movements on screen, to the point of moving the point pixel by pixel.

It's base on mouse speed and acceleration. At very low speed/acceleration, you'll have mouse precision.
At high acceleration/speed you'll have the "warp" effect.
In beetween: normal movements.

Both things allow for compromise beetween extreme precision, and the ability to move around the screen without making large movements with the mouse.

So you tune that once, to levels that aren't to annoying, then get used to it.