Aha, I see now that you have misunderstood what kind of data the Eye Position data stream contains. The Eye Position data stream gives you the physical position of your eye balls in 3D space relative to the screen, not the coordinates of where you are looking. So, if you position your head (and your eyes) exactly in the middle between the right and left edge of the computer monitor, the expected X value for the eye positions would be approximately for the left/right eye ball: 0 -/+ distance in mm between the center of the eye ball and the vertical center of the nose.
In other words, if you move your head to the right, the X value would increase, and if you move your head to the left, the X value would decrease. The Y value increases if you move your head upwards, and decreases when you move it downwards. The Z value increases when you move your head away from the monitor screen, and decreases when you move closer to the screen.