Why must our society call people with orange-hued red hair "carrot tops"?
The top of a carrot is not in fact orange. It is green. In attempting to reconcile this, I can understand that the phrase "carrot top" may not be so precise an analogy that it compares the top of the carrot to the top of the human (which is weird anyway).
However, I aver that all alleged to be "carrot tops" dye the tips of their hair green.