The pen cursor doesn't always exactly touch the tip of the pen. Ideally, wherever the pen touches, that's where the computer registers a stroke/mark/click, but that's not always the case.

One problem is the distance between the glass and the LCD - this problem is solved by optically bonding the display so there's no distance between the glass and the LCD, which most new tablets do. The distance between the tip of the pen and the pen cursor due to this issue is often called "parallax" because it stems from parallax.

Another problem is the accuracy and linearity of the digitizer. Accuracy is usually defined by how far the cursor is from the tip of the pen. Linearity is usually defined by the change in the distance from the cursor to the tip of the pen.

The edge drift issue is a problem with accuracy and linearity. The cursor "drifts" away from the tip of the pen. The distance between the cursor and the tip increases, and it also becomes unpredictable and unresponsive.

Parallax and edge drift are the biggest problems with Wacom EMR. Both of them have been solved, but Wacom and Samsung only implement the solutions in their high-end devices.