You are both right, the atom decaying is completely random but the distribution of the decays is still predictable.
If you have a random number generator your are relying on the fact that it is uniformly distributed and thus has no bias towards certain numbers.
Or if it is not uniformly distributed you would want to know the exact distribution so you can correct for it.
If you have an RNG that is treated as putting out uniformly distributed numbers but it is does in fact favor some numbers over others, that would be a defect that can cause problems/be exploited.
If you have a random number generator your are relying on the fact that it is uniformly distributed and thus has no bias towards certain numbers. Or if it is not uniformly distributed you would want to know the exact distribution so you can correct for it.
If you have an RNG that is treated as putting out uniformly distributed numbers but it is does in fact favor some numbers over others, that would be a defect that can cause problems/be exploited.