What your experiencing is most likely a limitation from the hardware.
There is a good tutorial on this subject at the Wacom site. But at the edges of the display, there are not as many reference points to statistically identify where the pen is. For example, when your pen is in the middle of the screen, it is like a dot in the center of a circle. The pen and digitizer work together and calculate the strongest signal point by scanning all adjacent grid co-ordinates.
This "edge" effect makes it imperative that recognition be done away from the sides of the display. Testing for both landscape and portrait is one of the first rules of software quality assurance for tablet apps.
What you are experiencing is most likely normal. Try writing closer to the edge with an app like Notepad and move closer to the edge and see what happens.
If you are experiencing problems anything more than a short distance from the edges, then you should look in other places for a cause and fix.
Kudos for your "test" though, not everyone takes the time to take a look at the issue.
Hope this helps,