I have a possible answer to Harry's question.
After reading more on the history of model cars and the Detroit Big 3, it seems to me that Jo-Han had greater access to the more acurate "last stage" clay or wood templates that the design teams made of the new cars prior to production (probably paid for well in advance...). Although Revell, AMT, and other model companies also made promo kits at the same time as Jo-Han, they all seem to be less acurate than anything Jo-Han ever produced, and this would make sense if all they had access to were older pre-production templates (since Jo-Han had already acquired all the more recent ones...). Imperfections in body size and proportion, missing badging, inacurate interiors, etc..., would commonly be the result of this practice.
What was missing in the early design templates would then have to accounted for in pictures, or eye-balling, or some other less-acurate practice in an attempt to get the kit as close to the real thing as they could. BUT, improper measurements would have already translated to the model kit because the base template would still be inacurate. And since the kits cost money to constantly redesign them, Revell et al. developed a "just good enough" approach to their product, knowing it was still a model kit people would purchase. Then, once modellers pointed out inconsistences in the kits, Revell et al. would sometimes correct the kits later on, like in an annual release or some other re-release point.
In summary, Jo-Han paid bigger bucks (OR, had contract rights well in advance...) to the automakers to get ahold of more acurate pre-production templates than the other companies, ensuring that their model car kits were more acurate in body shape and style than anything else out there. Now, if only they had spent more design time with the chassis of those kits....