Can you share your math? Radon does indeed have extremely nasty decay products, but they are very short lived (which is why they’re so nasty). They’re not accumulating in your attic.

I would suspect fly ash as a possible source if the house ever had a coal-burning furnace. Or maybe someone had a watch-painting operation or mixed radioactive glazes or paints there. Someone with appropriate equipment could do a much better job identifying the source.