autofocus - Why is fine focus adjustment sometimes necessary on DSLRs?

Many DSLRs have a "fine focus adjustment" option, stored in the camera per lens.

Why is it that some lenses need this?

Why do some lenses not project a focused image onto the sensor when the camera body has adjusted it to a point where it believes focus is correct?

I assume this is a side effect of NOT using the sensor itself to confirm focus, using the phase detection array instead...