Japanese Cabinet Secretary, Edano Yukio, explained that the atmospheric radiation levels being published are based on air samples not taken near the nuclear plant, and a computer model.

Edano said the radiation levels at the Fukushima Daiichi plant, are too high to take air samples there.  Air samples have been taken in surrounding areas. Then a computer program is used to make radiation level assumptions, based on the samples and possible wind speed and direction.

In other words, it’s a best computer guess.