Except when she obviously ignores one of the top dancers, then many "inferior" dancer might not invite her. The idea is: “If she feels too superior to dance with him, why should she put up with me”. An exception form the exception would be, if there is a known quarrel between the two persons and a third “inferior” dancer finally sees a chance to win the woman. An exception to exception of the exception ... . Well you get the idea, this are no laws but general guidelines or customs, like rules for clothing.