Never, ever take the measurements from the wheel well openings! I have seen up to an inch of differance side to side on many A-bodies. The center of the frame rails is the center of the car not the body.
Yes you will see that if you measure from wheel well to wheel well. All the driveline is offset to the passenger side of the car.
You measure from outer body panel to the other outer body panel and that gives you thd truecenter of the car not the drivetrain. The engine, trans and rear have a offset. Not sure what you are refering to being up to a inch off.