One assumes if there are processes needed in the world in which atoms are involved, that God knew. So the world would have started off with rocks and isotopes in place so that the processes could start to work. The forces and laws that existed at the time would determine how all the atoms behaved. We do not know what nature existed at creation, and afterward up till the time of Noah. There is no reason that I am yet aware of, or have heard, that the nature that existed was the same as what we have today.
Therefore that different nature would be responsible for a lot of the isotopes we have. It is not known that there even was radioactive decay in that time. So whatever processes were going on may have had nothing to do with any decay sequence radioactivity. We do not know what forces and laws existed.
All we know is what nature exists NOW. We see decay NOW happening, and science has simply assumed it was always like this. That is why the ratios represent great ages in their minds.