I've been perplexed about this too, and my attempts to research have led nowhere. It seems to be one species, but some get much bigger than others.
My best guess, based on what I've found is that there are multiple wild populations. Much like sulcatas from sudan, or certain wild population of the water monitors, some population grow larger frogs than others.
Dear Tom and others, You have been a lot of help to me raising my sulcata. I need help understanding anything about the African dwarf frog. A young friend of mine has a fresh water aquarium. He is very excited because his little African dwarf frog has been singing. We imagine he is calling for a mate. My friend has other small aquarium fish also in the tank. It is a beautiful little set up but what I have found out with raising my sulcata beautiful isn't always a good habitat. Do you have a care sheet out there??? Also do you have any good breeders you would recommend that I can purchase him a female? Thank you for your advice, AGAIN!!!