There's still a difference between the kind of radiation exposure that occurs during a flight compared to during a screening. My understanding is that radiation exposure during a flight is due to cosmic rays which are equally likely to cause damage throughout the body. The X-rays used by the TSA are at lower energy and are deposited almost exclusively in the skin. Since the volume of skin is much less than the total volume of your body, the radiation dose that your skin is receiving from the TSA is much larger than the dose during a flight.