simple reason. There are thousands of people from all parts of the world in that area. Sure, they might have been able to lie in the beginning but not now. Too many people outside direct control of government to hide any true discrepancy between official reports and reality of the situation. Different interpretations of the data do happen of course and governments always tend to favour more convenient interpratation but saying that situation is 'a lot worse' is definitely unfair.
Japan is not soviet union, you know... which did fail to hide true scale of the catastrophe in Chernobyl anyway.
By "fair" I mean "in my ill informed opinion".