Though there are ~20,000 genes in the human genome, only a select portion of it was a consequence of influence from each period of the human evolutionary timeline.
Further, much of the genome does not code for visible traits, and we've made a decision in our product to focus on genes that produce visibly appreciable traits for entertainment and informational purposes. Sometimes, modern science also does not yet know how genes produce our unique traits, so we cannot interpret them easily.
With Neanderthals, for example, there may not be a functional difference between many Neanderthal versions versus those from modern humans – we select for the interesting traits that are most relevant to spark a discussion. We hope you agree!
We write reports about traits when we have evidence that the trait can be predicted with a certain degree of accuracy based on genetics. We find these traits by reviewing many scientific papers about the genetics of various traits.