Because all the different rices are quite different plants? I think.

There is no white rice plant. It's all brown rice with the bran removed. Brown short grain rice doesn't have different starches than the same rice without its bran.

Here's an interesting article about white and brown rice characteristics:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8116841/

Seven different plants called 'rice'. For reference: https://en.wikipedia.org/wiki/List_of_rice_cultivars

> The cultivars listed in this article may vary in any number of these characteristics, and most can be eaten whole grain or milled (brown or white).

The article you posted backs up what I'm saying. Brown/white is a result of processing, not genetics.