They had different stone tool traditions:
Sileshi Semaw at the National Centre for Research on Human Evolution in Spain and his colleagues identified two H. erectus skulls at a site in Gona, Ethiopia. One was 1.26 million years old and the other dated back at least 1.5 million years. Unusually, the skulls were found directly alongside various stone tools.
Alison George, “Homo erectus used two different kinds of stone tools” at New Scientist
Skull fragments from two individuals were recently uncovered in Gona, Afar, Ethiopia, alongside associated stone tools—a rarity in archaeology. Even rarer is the discovery of stone tools hailing from two different technological traditions, a finding that’s upsetting a conventional notion that associates single human species with single stone tool technologies.
George Dvorsky, “Extinct Humans May Have Passed Down Stone Technology for Thousands of Generations” at Gizmodo
But why did we think they wouldn’t be? Isn’t there an underlying story here that is slowly being confuted (but no one wants to really discuss the history in those terms so everything must be treated as a big surprise)?
See also: Do racial assumptions prevent recognizing Homo erectus as fully human?