Kinda deflates your point about proper terminology when you don't use proper terminology
AGL stands for "Above Ground Level". It it the height of the aircraft above the ground directly below it. Not the height above the takeoff point.
What you trying to say above, and failing, is it's possible to exceed 400' AGL while flying away from a mountain while the height above the Home Point altitude can read less than 400ft.
You have presented a confused, wrong definition of AGL while trying to explain AGL.