[MrB]

Surprised the manufacturers could not get it right! Ed.

Perfection would most probably only be attained at a ridiculously high price point, Ed, and so it is astonishing how close they often are.

To make cameras affordable, their components are manufactured within a range of tolerances rather than to perfect accuracy. Many DSLRs use a separate autofocus sensor, whereas the image is created at the camera's image sensor. So it should not be a surprise that even extremely microscopic differences in distances between lenses and sensors (although within the tolerance ranges) might combine to cause slight errors in focusing, particularly for close subjects, and/or large apertures, when depth of field is already very small.

Cheers.
Philip