When you buy a new lens, you take it for granted that it will autofocus with 100% accuracy. And why not? Unfortunately, that does not always happen. Even the most expensive lenses, sometimes, do not autofocus with the accuracy that you may hope for. This is true across all manufacturers.
At times, the critical focus is a wee-bit behind the autofocussed point (Back Focus); sometimes in front of the autofocussed point (Front Focus). The normal way out is to send the lens back to the manufacturer for AF adjustment. This can be inconvenient, involves cost and you would be without the lens for quite a few days.
Some manufacturers incorporate a feature within their camera bodies that allow the user to fine-tune the autofocus. This is useful but a quite complicated and cumbersome procedure for most photographers. But the question is, why is it that some lenses do not autofocus as accurately as we would like them to?