actually my opinion lies in your statement . u hav already said that this expression varies fm country to country. actually this is not somethin defined in constitution! it just follows tradition. sometimes it depends on religion.
for example two hindus greet each other by sain namaskar. while two muslims r supposed to greet each other by sayin assalamu alaikum.
so people express their greetings to others followin their religion, social tradition n so on.