Japan's numbers are low but that doesn't mean the outbreak is under control there. There has been a very low rate of testing compared with other countries and criticism that the government has been denying the seriousness of the situation. In recent days the number of cases has increased and there is widespread concern that it could get worse. Incoming passengers from specified countries are being quarantined. While there hasn't been a lockdown yet, the Tokyo governor has made repeated appeals for people to stay at home which is the most she can legally do.
Japanese people tend to wear masks when they have a cold, to prevent passing it to others, rather than to protect themselves from disease. As some people who have COVID-19 are asymptomatic, it's less likely that these people would wear a mask. There has also been a shortage of masks in Japan, with difficulty finding them in some areas.
So in summary, Japan may not be an "outlier" for much longer.