Great work with this btw. I’ve asked people this question in a slightly different way: “what’s your background/ancestry?” because I’m fascinated by genealogy, history, politics, etc. though I can see the question can seem unsettling or disrespectful.
I wonder though if Asians get asked this question because more often than not they telegraph their ethnicity as “Asian” to whites, rather than as “Chinese” or “Korean” or “Vietnamese”, perhaps because they assume whites won’t understand the distinction anyway. Most whites of European ancestry tend to describe themselves as “Italian” or “Greek” or “Portuguese.” They rarely call themselves “European.”
It may just be that non-Asians need a little friendly training from the various Asian communities. I’m familiar enough with Chinese and Japanese names, for instance, to make educated guesses to a person’s background. But a lot of (white) people haven’t been taught or bothered to think about it.