>It's unlikely that the sterns actually tow out unless there is play in the beam to hull connection. The back beam is too close to the transom for that.<

Sounds like you never have measured your boat?
It has more to do with class of boat. Every class boat is different, etc.
I have measured '70s vintage Tornados, a '90s H20, a '80s TheMightyHobie18, & '70s H16.
Since the beam is so close to the stern, it is not linear proportional.
Torsion comes into play too, and the beams flex/twist a miniscule, etc.
The toe measurements also vary between the top of the rudder and bottom
due to some twist coming from somewhere as the main is sheeted in.
I never measured each boat to find out why, as the deltas as so small.