>>3984930AF detect basically zeros in from going over and under in distance/focus and averages the one in between. The mechanism also suffers to wear and tear so in final adjustments the camera correctly sends the signal how to adjust, but it's over/under by a tiny fraction due to loose components. It doesn't do a final check that the image is in focus, it just expects it to be as far as I know. That's why most modern cameras have the feature to slightly shift the focus to compensate for the lens wear. You can even print off a page for the calibration yourself and fix it. It's good to routinely do it. The issue starts to become bothersome when you're switching lenses frequently if your camera doesn't store lens specific data.
