Just because someone is from India, it doesn't mean they know everything about (incredibly diverse) cultures from every other country on the same continent. It's like asking an American of the culture of Brazil.
It's along the same lines as:
[Hears X is from country Y] "Oh do you know bob?"
whether he is indian or he only knows about Indian culture, it is a stretch to say "so you must know about Japan!"
Which is an entirely reasonable question, because if X from country Y is at the same party as you, then there's a good chance he might know your friend Bob. It's a small world, they say.