i'm not real sure on why grams. but logically it seems the easiest. most of your postage scales are either grams or ounces. and thats what the first dealers used probably. now all the digital scales and whatnot are all in grams. mine has the option of grams, ounces and something else but i don't know what it is. but grams is a weight measurement and that seems easiest for dealers and normal people. thats just what i think.