On Windows, the tablet event's position is scaled relative to the rect returned by QApplication::desktop()->geometry(), however, when a tablet such as the Wacom Cintiq is used as a secondary monitor, it has a different resolution than the first monitor. This causes an error in scaling the coords since the desktop area is reported to be different than the actual dimensions of the tablet screen. I'm not sure if this same error exists on other platforms as well.

The relevant lines are near qapplication_win.cpp:3492 (inside bool QETWidget::translateTabletEvent(...))

QRect desktopArea = QApplication::desktop()->geometry();
QPointF hiResGlobal = currentTabletPointer.scaleCoord(ptNew.x, ptNew.y, desktopArea.left(),
                                                              desktopArea.width(), desktopArea.top(),
                                                              desktopArea.height());