They may be the same people, but the obviously do not reside in the same geographical locale. The reason why you cannot tell the difference is because of many different factors, one of them being geographical (if they live in the same place, then its likely they'll develop similar skin tone), the other being marriages between these different people. I'm sure there are many other factors. Just look at people from Assam, they're as fair as Europeans! And the north east has seen little migration, so we can see that people there are pure locals