Okay I think I understand what your saying. The dV' I solve for is the change in velocity of the contact point which is linear. It's an instantaneous velocity. But, when I apply the forces to the body that contact point doesn't have a linear trajectory since I'm applying a torque.

muleax said:
This effect should be less noticeable when term r x w is small: smaller penetrations (usual case) or, for example, if you rotate the box on 45 degrees and collide with horizontal ground.

Yep, I've noticed this is to be true. It makes sense, if the box was rotated 45 degrees then the contact point would have a linear trajectory.

I appreciate all the help you've given, thank you!