I think frequency of discovery and where they happened to be located would affect that. There may be just more discoveries along that particular area because it’s a hot spot where fossils happen to get exposed. And here is where the model could be flawed a bit, there may be future discoveries later that completely change this map and frequency of discoveries and locations. They just haven’t been found and put on the map yet. They are elsewhere too, just buried deeper and still undiscovered. Someone will dig a pipeline or build a road and more will be found.
A lot of people don’t realize that fossilized ape remains are extremely rare. I think the first ever chimpanzee fossil was only found a decade ago. Simply put, apes or humans don’t generally die on muddy banks to be covered before the body is scavenged.