It is very common in Japan to conceal facts of deaths of elderly relatives to defraud government and keep collecting their benefits. It might have contributed to their health statistics to some degree.
On the other hand I wouldn’t discard ‘Japanese diet’ as well. The most popular cause of death is a heart disease and Japanese food is certainly less stressful for heart than diets of people from countries which aren’t known for long lives of their nationals.
Causes of death enter popularity contests?! Who knew? :)