Different lines of the same gene.
Ralph Davis once told me even though they are co-dom the other snake, be it normal or morph, that it is paired with can effect the look of the offspring. So now a days we are seeing a large variety of lessers and butters not only from their lines but from past breeding. Back in the day when you paid $30,000 for a lesser male, most breeders would breed it to as many normal females as they could and the offspring picked up traits the pairing be it clean or dirty, browner or yellower, etc...
All Lessers come from Ralph's original lesser and butters were different lines brought in from Africa that looked different until they were all proven compatable.