I realize that the only Hispanics who get a boost are Mexicans and Puerto Ricans but aren't Hispanics as a whole underrepresented in top law schools compared to African Americans. For example Harvard has 11.8% African Americans and 7.3% Hispanics while the population of the US consists of 12.4% African Americans and 15% Hispanics if the point of URM is because they are underrepresented then shouldn't Hispanics (whatever the origin), at least at Harvard, get more of a boost than African Americans? *Statistics at other top law schools also seem to hover around these numbers.
It depends on the focus of increased representation. If you simply say, "this is the rate of Hispanics in the population vs the rate of Hispanics in law school," then your assertion would be correct. However, I think law schools are sensitive to the designations made in the US Census since this where all "representation" claims originate. The Census now accounts for Hispanics of different national origin. Therefore, law schools look at Hispanic with a finer level detail than they otherwise would and determine which sub-groups really are underrepresented and which are not. Knowing this information, it would make no sense to give "all Hispanics" the same URM consideration.