Comment by BetterGeiger

It is a myth that radon collects in "low points", concentrations are low and it diffuses as the molecules bounce around. However, as you said radon is entering into those areas primarily, which means it will naturally be higher in those areas. Secondly basements often have poor ventilation, which is not to say that the radon cannot otherwise escape, but ventilation can disperse it much more rapidly than diffusion.