Cityspeak, the slang used in Shadowrun, has a lot of Japanese peppered in there. Given the large number of Japanese corps with offices in Seattle, not to mention the recent Japanese prescence in CalFree, I think it's safe to say that there would be a much, much higher Japanese population in North American than there is currently. So yeah, you'd find a larger proportion of Japanese descent shadowrunners than you might expect given current population statistics.