Ok look at it another way.
You have two objects that form two parallel lines, or slopes when you get the angle between the two.
An axis of points for each.
Suppose one object goes up or down its axis/slope, then angle() would change.
How would I get the new angle for the few pixels difference?
I mean in theory I could cast the new points using trig, and angle() from those points but that seems like overkill.