POC is homogonizing because it's reducing the space from N strata to 2: white and non-white, blending n-1 strata.
Per wikipedia:
> Stratification is the process of dividing members of the population into homogeneous subgroups before sampling. The strata should define a partition of the population. That is, it should be collectively exhaustive and mutually exclusive: every element in the population must be assigned to one and only one stratum. Then simple random sampling is applied within each stratum
Using this definition, white and non-white are mutually exclusive, while location of origin forms a different axis for comparison.
In more social sciences, you might look to intersectionality for an explanation.
[0] https://en.wikipedia.org/wiki/Stratified_sampling
[1] https://www.nasponline.org/resources-and-publications/resour...