I think the game is using the value SupportPoints out of table claims somehow.
Seems that the "field strength" decreases by distance to the monument (see the "border" area is not straigt)
I have a small point plotter for some other reason and put in some example values. It generated a similar behaviour.
I am using a formula like "field strength" = ("max strengh" - "dist from center")
2This or a similar formula would explain the shape and the position of the border line.
But what I cannot explain is the complete shift of the left claim area to the left