The screen setup assumes that the EyeX Controller is mounted at the bottom of the monitor. If you place it someplace else it will affect the accuracy of the eye tracking, and there will be skewed offsets. Depending on how far away from the optimal position and angle the EyeX Controller is placed, the calibration process can make up for some of the offset. But as soon as you start moving your head around (away from the position you held it in when doing the calibration), you will get noticably worse accuracy.
When the EyeX Controller is mounted, setup and calibrated as intended, it allows for you to move your head around and still have good accuracy.
You can always check how good the eye tracking is by opening the EyeX Settings panel, and on the Eye Tracker tab click the “Test Eye Tracking” button.