To be clear this has nothing to do with using tilt data to draw strokes. This refers only to the effect of the physical tilt of the pen on pressure readings.
What we find is that the physical tilt angle effects how pressure behaves. When holding the pen perfectly vertical the the pen is most sensitive to low pressure. And the more you tilt the pen the less sensitive it is to low pressure.
This is an expected behavior of EMR pens. Because the more the pen is tilted the more force is not directed through the pen to the pressure sensor but to the sides pen instead.