Acer c110 pen calibration problem

I have brought a new acer c110Ti convertible tablet pc a
few days ago. I am very happy with the pc except i found
that I need to calibrate the pen quite often.  No matter
how hard I have calibrated the pen, I found the pen being
mis-aligned especially at about 1 inch away from the
lower corners.  Is this normal?  Is this a hardware
problem or design bug? How can i fix it.  Does anyone
come across such problem in the old c100 series or other
brands?