A White LED consists of a Blue chip and a yellow phosphor. The blue photons generated in the chip can either:
1.travel through the phosphor layer
2.In the phosphor layers it can be converted into a yellow photon.
Those two colors combined will generate white light. With a thicker phosphor layer more photons will be converted to white light, thus changing the color point of the LED. Also the wavelength of the blue photons influences the color of the emitted light. Therefore controlling the phosphor layer from LED to LED and over the chip minimizes the color variation.