Bias and prejudice
Double standard is the trademark for people of prejudice or bias. For people of double standard, they really have no standard, no principle. People without principle are often observed in uncivilized barbaric tribes. There is really no rule, no law, no principle, no morale in barbarism.
Then why there is double standard in the first place? Double standard is product of barbaric instinct in civilized environment. Standard or principle is guidance of fairness for civilized people. To cover up their barbaric nature, people of less civility create one standard for themselves, another for other. Barbaric people pretend to be civilized in civilized world.
Though no culture is immune from double stand behavior, double standards are observed more often in some cultures than others. Such cultures are often newly civilized from barbarism. Barbaric people are hastily thrown into civilization and their barbaric instincts are still quite strong. To fulfill their barbaric nature, double standards are created because barbaric people really do not want to be fair.