The reason your thighs may tan faster than your calves may be due to the production of melanin in your body. Your claves, when compared to the rest of the body and especially your thighs, do not produce the same amount of melanin. This may be affecting your tan. This lack of melanin will cause the skin to be thicker around your calves. This means that the UV light from the tanning bed will not be able to penetrate the skin as easily as the rest of your body.