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...
Lenses and cameras are real objects and have manufacturing tolerances which are generally good, but you'll almost never end up with a lens or camera whose specs precisely match the design. No two lenses and camera will be absolutely identical.
A lot of the time (most of the time) these differences are too small to worry about in practical shooting (even professionally).
But sometimes you get e.g. lens and/or camera (or more typically the combination) have tolerances which mean the result is just too far outside what you need for that perfect shot.
So to compensate you can set microfocus adjustment.
There are a lot of complications to doing that properly and, IMO, most people who use it have no idea what they're doing. Just to diagnose the problem properly you have to be certain your technique doing the formal testing is spot on. I've seen a lot of amateurs make things worse trying to correct focus issues which are really general technique related and not this issue I've described.
Fine focus exists because sometimes autofocus does not work as intended.
DSLR use the image reflected on the mirror to focus, usually the difference between this and the image projected on the sensor is negligible but there are cases when it is not.
This flaw is only observable on wide apertures when the depth of field is shallow. Although there are extreme cases when this is observable even on narrower apertures. Sometimes using live view can help with this issue.
I don't know exactly why is is that some lenses behave differently than others, it is most probably manufacture problems. Still the culprit of back or front focus is mostly a body not a lens.
Fine focus allows you to compensate for these small flaws.
