Fred,
Let me answer with a comparison.
For most of your lifetime and mine, shotgun slugs were defined as full bore, weight forward soft lead designs that were safe and reasonably accurate from choke bore shotguns. Then along came BRI, 45 and 50 caliber sabot slugs and rifled barrels. While initially offered by smaller manufacturers, these are now generally accepted as shotgun slugs and the long accepted "traditional standards" for shotgun slugs are now just one of many types available.
Buckshot today, while still consisting of round lead pellets, have greatly improved performance from what was the norm in the middle 1950's. But performance aside, with rare exception there are no required standards for buckshot sizes. SAAMI standards are all voluntary, even for member companies. Today manufacturers produce buckshot for handloaders in a range of sizes, (of their own designation), both larger and smaller than traditional. Available factory loaded non -traditional buckshot load ranges include, .20", .40" and .60" pellets. Some are made from hard non-toxic metals, some are pure lead minimum penetration home defense loads, some are hardened lead for maximum penetration. Indeed most traditional buckshot sizes are simply smaller today than in the 1950's, in order to fit in specialized shot cups.
All are buckshot - today.