Water infiltration depends on the amount of water and type of soil and the drainage designed into the basement. e.g. layers of concrete and rows of shale/rocks for drainage. Not all basements are equal. The amount of radon depends on the quantity of uranium and thorium in the soil/dirt. So you are right they are not always related but the mitigating controls can be potentially related. Some construction companies just focus on maximizing profits in my opinion and some individual home builders will use construction dirt that contains low or no uranium or thorium.