Xbox 360 controller calibration (possibly all joystick-like calibration in Win7) is half-assed. One analog stick's Y-axis easily reaches and slightly exceeds the maximum range, the X-axis doesn't reach the limits. So the movement range will be slightly egg-shaped or more precisely like a circle with two flat ends.
And when the centering is off, calibration doesn't fix that. It always puts the center in the middle of both measured extremes, even if the actual center is still not where it's supposed to be because of the stick's imprecise centering. If this worked better, I could minimize deadzone for the analog sticks wherever that option is available in a game. ... Hm, it's probably not available in any game, since this is all oh-so standardized and optimized.
The Xbox 360 controller's precision is merely that of a joystick in the same price range. No positive surprises in value-for-price here.