the analogsticks of your controllers are not perfectly centered. thats physically impossible. they are sending constantly their position to the console. the deadzone blacks certain spot in the center out, if you have your thumbs on the sticks, you correct the drift, but if you use a xim then its annoying. in close quarter its not as @#$% as in mid- or long range battles. every controller drifts slightly different.
it would be cool if we had a slider in the option menu for fine tuning... not just 3 steps