About the ”Y Ratio vs. X Ratio” when using ADUX 1020

Hi all,
I would like to ask about ADUX 1020.

Using the Optical Gesture Evaluation Tool,
I'm evaluating ADUX1020-EVAL-SDP(EVAL-SDP-CB1Z and ADUX1020 MCM board).

I checked the graph of ”Y Ratio vs. X Ratio” on the Graph XYI tab.

Even if I move the finger horizontally,
As shown in the attached image, the locus is a circular arc.

The same phenomenon occurs on both two evaluation boards that I have.

May I recognize this as a specification?