These blotches of light and dark skin are quite common among Asian skin, and are wrongly thought to be due of lack of calcium. They mainly occur due to sun light exposure which causes uneven distribution of melanin in our skin cells. Its also called blotchy pigmentation.
Moisturizers and sun blocks can help, but the disorder itself takes a few years to clear permanently. When these spots appear, they can be treated with a topical skin lightening cream or lotion.