It isn't rocket science, it isn't binomial nomenclature or chemistry.
"c"omputer "R"ole "P"laying "G"ame. It was a differentiation from jRPG that were popular on consoles.
cRPG became synonymous with isometric party based adventures with western cultured fantasy. jRPG became synonymous with isometric hero based adventures with east asian cultured fantasy.
any further differentiation is just postulation and preference. those were the ONLY two designations until cRPG nerds couldn't agree that both Diablo and Baldur's Gate were RPGs, which is where the aRPG came from to describe Diablo.