The standard categories used by USA governmental forms that ask for a voluntary disclosure of race or ethnicity include the following: Black or African American, White or Caucasian, American Indian or Alaska Native, and Asian American and Pacific Islander. However, on the forms used for the Affordable Care Act (ACA) Marketplace, the Asian American category is subdivided into Vietnamese, Japanese, Korean, and so forth (with "Other Asian" as a catch-all), while the other categories remain unchanged, save for the separation of Native Hawaiians from the category "American Indian and Alaska Native" and the inclusion of categories for Samoa and Guam.
Dividing the categories provides more granular information about which healthcare Americans of various groups have, so that would be one obvious reason to separate them. After all, there may be sizeable differences between types of health insurance and outcomes between Chinese Americans (average household income of 85,424 in 2019) and Myanmarese Americans (45,903). However, this would seem to apply to the groups that are not distinguished as well: presumably, healthcare outcomes might differ between Americans who are Apache (average income 28,745 in 2014) or Chicksaw (average income 49,963) or those of Bolivian descent (72,699) vs Guatemalan (49,584).
As such, what is the reasoning behind the race and ethnicity categories used on the ACA marketplace?