Radioactive atoms, in the aggregate, don't have a half-life. If you measure the nuclear radiation coming from a heterogenous glob of radioactive material, and it takes N hours to drop to 50% of that level, then it will take more than N hours to drop to 25% of the original level. The original material had both shorter-lived substances with a half-life of less than N hours and longer-lived substances with a half-life of more than N hours (and possibly non-radioactive elements as well), and since the former materials decay faster than the latter, the ratio between them falls until the latter materials dominate.

I suspect the same process holds true for facts.