Grimulkan,
that matches what I find and my understanding of what others find. Provided you are 0.5 - 1cm away from the edges, the calibration is consistent. Closer and it goes pear shaped in a non linear manner.
I have no idea why as it doesn't happen on my wacom drawing pad. It is unfortunate that more points may not help, it wold have been more use to mask the screen so you do not find these non-linearities, maybe I'll get some masking tape
