Snoozy, the dentist covers your chest with a lead apron because your teeth are being examined. Since approximately 70% of all x-rays are for chest problems, the chest usually isn't covered. It's just a protection from scatter radiation.
JeffT is right. The cost of a malpractice suit is what causes a lot of extra examinations while at the hospital.
Another factor that can cause multiple examinations is obesity. Techs try to set the amount of exposure at the lowest possible amount. Excessive weight means that higher amounts need to be used in order for the radiologist to be able to properly read the x-ray. Sometimes that means the techs need to take an x-ray a number of times, progressively increasing the amount. A friend of mine told me about one woman that came in that needed 11 x-rays done before a readable picture was taken.