Comment by stubish

The statistics (in the sample data) becomes the stereotype. If 99% of your samples are white doctors, maybe you will get a non-white doctor if you ask it to generate a picture of a group of 100 doctors. But if you ask it to generate a picture of a single doctor? It will generate a white one, 100% of the time, because each time the most probable skin color is white. Unless we tell it to inject some randomness, which is what the prompt is doing.