I'm guessing it might have something to do with the "impulse curve" of the radiation. Atom bomb would look like a spike with exponential decay. This study looks

I'm guessing it might have something to do with the "impulse curve" of the radiation. Atom bomb would look like a spike with exponential decay. This study looks at constant low-grade exposure. It might be that a single main radiation event (if it doesn't acutely kill or sicken you too bad) causes an inflammatory response and increased mutation detection, while constant low-grade provides many more opportunities for errors to slip through the cracks.