Answer

The issue is a hardware limitation of graphic adapter.
In Clone mode, the displays may have different refresh rates and/or scan-line characteristics but the graphic adapter could match only one of displays at a time. Therefore, a thin horizontal line of tearing occasionally appears on the other display because of non-matching refresh rates, or simply lack of sync between the video adapter and display.
P.S. This phenomenon is called “Video Tearing” and it is a visual artifact in video where information from two or more different frames is shown in a display in a single screen draw.