In the PCB editor, set units to mils. Then open Set Board Outline Dialog. Set X & Y position to 0.0 and Width and Height to 500. Close dialog. Now open the dialog again. X & Y position will be -20, and width and height will be 540. The dialog increases the boundary by 20 mils all the way around. When changing to the correct values, the board outline is correct, but opening the dialog on an existing outline will always enlarge it by the 20 mils on each side. The same is true of other units, but the amount added is always 20 mils. This bug appeared sometime in the last 12 months I believe. I don't think it was always there. It has remained through several version updates, and is still in the latest version.