Because being an immigrant is not inherently under-represented in medicine. Were you one of the major chunk of medical student's whose parents were wealthy prior to moving to the US and now use said wealth to send their child to a USMD school? Because that is really common.
Or were you a refugee from the genocide in Cambodia a few decades ago? Or the Arab Spring? Or escaping the central American Cartels? Or what have you. Those kinds of applicants are not explicitly sought after for any inherent URM status or traits they posses, but they do tend to be successful applicants because they have genuinely interesting and/or terrifying stories that make them stand out and make them memorable.
Immigrants overall (south Asian, Nigerian, ME, maybe East Asian) are probably overrepresented in medicine compared to their fraction of the overall population. Certainly so in residency and beyond. Latin American, Afro Caribbean immigrants underrepresented and treated as such IME.
Low SES is a kind of URM in and of itself. We like come from behind stories. But kids of immigrant doctors or engineers? They're already born on third base.