Are Japan officials underplaying the radiation levels? Should people evacuate?

Japanese government told people that live within 18 miles of the nuclear facility to stay in their homes or evacuate. Yet confidently stated that anyone outside the 18 mile range was safe to leave and come as they may.On the other hand you have U.S government telling any Americans within 50 miles of the facility to jump ship and get out.

Tokyo, which is roughly 180 miles south of the Fukushima Daiichi plant says they are showing radiation levels at a slight increase. Let me restate that last line...... there is an increase in radiation 180 miles away from the disaster site. Yet and still Japanese government says anyone 18 miles out...(10% of the distance from the plant to Tokyo) would be completely fine to go on about their lives like everything is just fine.

Is Japan underplaying the severity of this event to avoid pandemonium at a time when things are already about as bad as they can get?