Westerners are considered expats, ‘3rd worlders’ are considered immigrants.

I noticed that in america, we call foreigners, specifically those from “3rd world” countries; immigrants.  Even if they are just there for a vacation, visit or work. However, a westerner abroad in any country is considered an expat. Why is that? Truth be told, I am an american and  I am indeed an immigrant in a foreign country. Why am I called an expat, but a chinese person in america, even just going to school or business contracting is considered an immigrant?  Funny psychological business going on here……