Radon decays into other longer lived radioactive elements, mostly isotopes of lead, which then condense down as dust and cause more problems than radon itself. So it might happen that these cumulate in the attic if circulation is just right.
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.
