Hi kangwang,
There will be some fluctuations of the eye position as the eye tracker’s view of the eye changes as the eye is looking in different directions, and there may be some stray points in the stream of data. Even when holding the head and eye-gaze fixed, there will be small movements in the eye position data. The data is “raw” in that sense – there is no smooth filtering of the eye position data stream. The eye position data is given in the unit millimeters but it is not millimeter perfect.
This means that the eye position data stream is mostly usable for relative positioning and averaged movements, rather than absolute positioning.
If I run the Eye Position sample in the Unity SDK (Unity 3D is downloadable for free), I can see these small fluctuations, and some stray points as I look to the far edges of the screen. But all in all, I perceive the two spheres representing my eyes in a 3D to follow my eye balls quite smoothly.