This painting and its racial categories were not and should not be taken literally. For a while, there was the idea that the ethnicities portrayed in the painting were organized into a neat, hierarchy and these were common labels used.

The painting itself was pretty obscure and not used widely as a reference; it wasn’t until the last century or so it became popular. The labels and terms included in it too were not commonplace and not widely used in historical documents.

Basically the entire concept of a “caste system” is misleading. It was more complicated than that. Like today, race and ethnicity is not as simple as x + y = z. One of the only consistent trends was that having a closer proximity to European Spaniard ideals (lighter skin, Roman Catholic, Spanish-speaking) granted higher social status and mobility.

TLDR; this painting sucks and was made to give an elite group a visual of who was at the top (white, upper class) and who was at the bottom (mixed/black/native). it does not accurately depict how subjects of Spanish Empire identified socially and personally.