How can Neanderthal DNA be so uniformly dispersed in modern humans?

In 2010 scientists discovered DNA present in Neanderthals and modern non-African humans and determined that this was the result of interbreeding between Neanderthals and modern humans. However, if that were the case, there would be a much less homogenous distribution of those supposedly Neanderthal genes. If the average was 4%, then many modern humans, including some with Neanderthal ancestors would have 0% Neanderthal DNA and many would have more than 4%. Nature loves diversity not homogeneity. I think this supposed Neanderthal DNA is a relic of the common ancestry of modern humans and Neanderthals that may have difted out of the African gene pool or may actully be there now but as yet undiscovered. Please cite any other example of admixture that resulted in a homogenous distribution of genes.