Perhaps to assist end users that hold the tablet at an angle to contend with the parallax created?
(Screen thickness + angle of view create a perceived shift in the true point of contact)
good point, you'd think that would be the case but from my experiments (trying to be clever by offsetting and changing the angle of the pen) the only way the parallax idea might
work is, if your eyes were about 3 inches away from the screen, i.e. closer than the top of the pen.
i'm being facetious, of course, but looking around the web it appears that other people have come to the same conclusion. the calibration routine on the device only makes matters worse - even when using a method of using a 16 point grid rather than the 4 corner grid, it's all worse that a simple click of the Reset button.
the calibration system is so wrong it just doesn't provide a consistent "offset" between pen and cursor. a single corner area might be off, or all corners are off in different directions. honestly, it's a total joke.
judging by some of the other comments i've found about the bad calibration it seems that some users are happy to forgive the machine. instead they'll enlarge the scroll bars or gui buttons, or simply say that watching the cursor and not the pen tip is the important part, i.e. they don't care that in some places of the screen, cursor and pen are off by a few millimetres. they obviously need to just hit the Reset button and everything will be fine instead of just lying to themselves that everything is acceptable.