Typically mutations occur during recombination periods (either cell replication, or reproduction events), this tends to allow faster replicating organisms to mutate faster. Given this, I would expect that the rate would be different based on the particular organism. Also, the mutations talked about are "sustained" mutations. There may be many transient mutations that were reversed/repaired during that time-frame. As humans do not evolve necessarily on environmentally based pressures (thank you, society), the incidence of sustained mutations may be lower than other closely related species, as there is no selective pressure to maintain them.