Ha ha.
So you pulled out the “that’s just dumb”defense….
The radius from ball to center of axle isn’t imaginary.
It is specifically defined for every trailer and the error being described in this thread is that radial miss alignment on that circle.
The axis defined is 0 degrees on square to the center of a line drawn between the contact points of the tires.
The axles are not in fact squared by necessity to the trailers frame.
What would one do if the trailer is an elliptical in shape to some degree, made with the frame elements out of parallel or any other permutations of shape.
What happens is a mechanic does in fact square to some elements of the trailers frame and the rig still crabs.
This is due to not having established the critical geometry of the coupler having minimal radial error.
Forget the frame- it is perfectly meaningless.