Sometimes I think it's more of a issue with the cars not always being built exactly the same. They should be but I know from experience that I've seen quite a few little differences between the same parts. For instance the front leaf spring hangers on my car have the spring bolt hole drilled nearly 3/16" off from every other A-body hanger I've measured. All the others have measured the same but mine has the hole almost 3/16" higher which lowers the car slightly and changes the pinion angle a little