If your monitor weighs less than 11kg, the monitor arm and gas spring mechanism will not be able to counterbalance the weight correctly and the arm will always rise upwards to its highest position, regardless of any tension adjustments you make.  Similarly, a monitor that is heavier than 20kg will cause the monitor arm to slowly fall to its lowest position. For monitor weights outside of the specified range the gas spring mechanism cannot be set to properly counterbalance the monitor.