Perhaps because if they are a self the differences would be minute. Often that results in a garden visitor saying 'I have one just like it' and going home and labeling an iris with a wrong name. Then when grown side by side, or when blooms are cut and vased for an event and placed side by side, people will say they don't know which is which and both become a NoID. Some peoples thought processes are chaotic and this results in chaos. The garden does not like chaos.
