It is caused by the arc length, and the swirl of the plasma stream. The arc length can give you either a positive or negative bevel if you are too close or too far from the base material. Most plasma cutters require you to move counter-clockwise for ID cutting and clockwise for OD cutting to minimize the angle of the bevel. This keeps you on the correct side of the swirl. The type of gas mix can also have an effect on the bevel. All plasma cutters will leave a bevel on one side of the kerf; precision or high definition types will have 1.5 degrees of bevel angle. You will have to cut thicker material to see it visually. My information may be somewhat dated, as I have been away from CNC plasma cutting for two years now; processes sometimes change rapidly. Regards, Brad