Why recalibrate?

[Mark Sirota <msirota@isc.upenn.edu>]

Brent DeWitt wrote:
> Since I've been dumb enough to forget a couple of times; Byron, please
> add "UP" to the end of the calibration button labels on the Palm
> Calibrate program as a reminder (ie "Left Side UP").

Question: Why is it necessary to calibrate more than once?  Should I be
doing this regularly?  Why wasn't my original calibration just fine?

Mark