The difference is based mainly on average body weights and calorie intakes.
Selenium is part of the body's antioxidant defense system.
The method used to estimate the requirement was the amount needed to maximize the blood level of glutathione peroxidase, an antioxidant enzyme. The tolerable upper limit was set at 400 micrograms per day. This seems to be a very conservative estimate because most data suggest 750 micrograms might be the level where toxic signs show up. It is too early to recommend supplements for this purpose. Americans already have the highest average blood selenium levels in the world, New Zealanders the lowest, and cancer rates in the two countries are similar.