Nikon D300 Complete Manual page 396

V1.02
few factors that come into play that can keep the camera/lens
system from being dead-on all the time. Just to name a few:
temperature variations, humidity variations, slight mass
variations, slight autofocus motor variations, and most
importantly slight variations due to factory alignment
procedures (which have a built-in tolerance). Indeed, it's
usually this last one that causes lenses to consistently focus
not quite to the proper point.
The phase detection system relies upon moving the lens
elements a specific distance in a specific time. But what
happens if it's gotten really dry and hot and the cam that
controls a lens' internal movements has gotten slightly stiff?
Well, the focus element may not move quite as much as the
camera expected it to in the autofocus time allotted. ED
lenses can focus slightly differently at different temperatures
due to the glass used in them.
But it's more likely that the mirror and autofocus sensor
alignment and ever so slight variations in mount thickness are
the real culprit for most "doesn't focus quite right" scenarios.
The AF sensors at the bottom of the camera have to be at the
precise same distance as the sensor, perfectly perpendicular
to the light, and the mirrors that pass the light to the AF
sensors have to be absolutely perfectly aligned, too. And
those mirrors are flipping up and down every time you take a
picture.
So while it's rare, it's not unusual to find that a lens you have
doesn't focus perfectly on the optimal spot or a camera that
needs a bit of tweaking to match all your lenses. Most new
lenses you get these days—especially AF-S ones—are usually
dead on, but almost every Nikon user has found one or two of
their lenses don't quite focus where they want it to. If that's
consistently in back of the subject, we call that "back
focusing" and if it's consistently in front of the subject we call
that "front focusing." In general, when a lens is off, it tends to
be off consistently in one direction.
Thom Hogan's Complete Guide to the Nikon D300 Page 396