Or: Race is a rough proxy for breeding populations that have been separated for long time, and subject to different environments' selection pressure. Over time, selection and blind luck build up all manner of small differences, which both observably exist at the surface (so why not inside the body) and that you'd expect to exist from basic evolutionary principles. You'd expect a Norwegian forest cat, a Saharan desert cat and a Burmese to be different in innumerable small ways because they grow up in totally different environments. You'd also expect there to be a lot of overlapping, well, catness to them all. Lions purr, after all. There's nothing complicated about any of it, humans just tie themselves into knots when it comes to humans in a way they don't when it comes to cats.

With that said, the simple explanation is that the AI picks up on these small patterns in a way humans don't. The brain and neural networks are fundamentally pattern-recognition engines. The AI is just seeing something we don't either notice or can't see.