Since the center of the meat's thickest section is farthest away from the oven's heat source, it is the last part to cook. Even if the thermometer gives the desired temperature for a thinner part of the meat, the thickest segment may not be done. Bone is a better conductor of heat than meat. If we take the temperature reading next to a bone in the center of the meat, the temperature reading will be higher than it would be for center flesh farther from the bone.
Fat is not as good a heat conductor as meat. Placing the thermometer point in a pocket of fat, therefore, gives a lower reading than it would for the adjacent flesh.