How are you measuring it?

WAGs (wild A55ed guesses) follow:

Perhaps the measuring point is bent. Perhaps the frame is bent. Perhaps a shock has pressure and the other doesn't.

Perhaps it is related to the front end. The front springs and roll bar make the front end much-much stiffer in roll resistance so perhaps a bent front end can make the back end sit crookedly. Try measuring it from the tips of the headlights to the top corners of the windshield, as well as the diagonals. (Although I'm not sure exactly how square the car was rolling out of the factory) The exact measurements aren't critical - just any differences side to side. Finally it could be a sagged front spring.