Why do some stone yards group stone into groups 1 - 5?

Some stone yards or mass-production fabricators who install stone in tract homes or commercial homebuilders use a system that gives their homebuyers a choice of 5 price ranges.  Since there are over 1100 types of slab stone available, all of them varied in size and price, from $6 to $125 per square foot, the 5 group system is a way to simplify them. True, the system is over-simplified, but it is a way to help customers get a basic idea for comparison.