I'm currently reading about half lifes.

The thing with half lifes, things don't stop decaying when their half life is up. They continue to decay. If something has a half life of a million years, then after 7 million years it retains 1/128 of its original energy value and continues to decay.

So the plutonium and the potassium have *mostly* decayed, while the uranium hasn't even lost half its energy yet.