You can actually produce different accuracy regions by calibrating the stylus differently. Sometimes you can get half the screen working accurately, sometimes the center region and not the edges and so on. Different people angle the stylus and screen in different ways, and result in different calibration, as I think there is a small gap between the surface of the screen and the sensor.
You can try "missing" the calibration points and come up with a calibration setting that best suits your purposes, although there will always be some points that are inaccurate. I don't know if this is because the calibration points are too few, if there is a bug in mapping the points to actual values or a bug in the driver itself. In any case, it should be fixable by software.
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