For years, the standard definition for URM has been:
Native American (on a tribal registry)
Puerto Rican (Mainland)
Mexican-American (US citizen/permanent resident)
African-American (US citizen/permanent resident)
The definition has nothing to do with the number of applicants in any grouping. It has to do with American history, economics. and historical biases in admission.
There are a large number of different "minorities" in the US now. Some are over-represented in medical school these days.
The definition arose before the big wave of immigration which started about 1960 and has since accelerated.
Admission to medical school in this country had never been bias-free. Economic and social class has been a filter. Subsidized loans for medical students did not exist. There was systematic discrimination against Jews, Catholics and other non-WASP (White Anglo-Saxon Protestant) groups.
Anti-semitism was first broken about 1955. Beginning in the 1960s the barriers to the other groups began to fall. For various social and economic reasons, the numbers of those under-represented-in-medicine groups have not grown greatly.
Admisson to medical school has never been and is still not based only on grades and MCAT scores; if it were, it would be a disaster for the public. What has changed is that irrelevant criteria, such as ethnicity, religion and unsubsidized access to large amounts of money are no longer significant. On the other hand, on the lead up to a college/medical education, social and economic factors may still be significant. In that sense, it is still not a level field.
Unfair? What exactly would be fair? The answer is not as simple and obvious as it appears to many. Since even the college/university premeds attend is a factor (for several reasons beyond the obvious one of "prestige")--in admission, how do you propose to level that field? Some colleges have excellent, proactive advisors who have good contacts, while, at the other end of the scale, others have no advisors at all.
Life is never as simple as we would like it to be.