Interestingly, If you set the max at 50 as stated it will slide all the way but will not put out more than 50. However, if you attempt to sliee the tune power to more than 50 it will bounce back to 50. Hmmmm, feels like a bug to me.
What is being displayed on the screen when max limit is set to 20 and it shows 100 on screen you are seeing the percentage of the max limit. If you set it to 50 on the screen you should be at 10w out with the max limit set at 20.