Years ago, at one of the annual ANA conventions, the slabbing people took a look at our rapidly growing hobby and said--WOW, we could make a lot of money off these idiots! and that is how the chip slabbing started. They started getting people to slab their chips, but didn't have a grading system in place, so just made something up.
Thereafter slab grading has never been legitimate, and probably never will since other slabbing companies have now also gotten into this market with their own grading standards.