Whiteness is not an objective quality of people but a socially constructed category that expands or contracts depending on what is needed to maintain in group dominance.

When the in group is weak, you’ll see more people being considered “white,” and when it is strong, the tests will get stricter (looking not only at skin color, for example, but at ancestry). It’s really just a question of how confident racists are that they have sufficient clout to exclude a particular group without undermining their exclusion of other groups.