What's the need for two different threads on one screw?

The lower coarse thread pulls the soft composite material up and makes a "bubble" of it on the surface of the board; the upper reverse threaded fine threading pushes that material back down into the board for a flush finish.

The upper thread because it has a different pitch works to draw the composite deck board down more tightly to the material to which you are screwing it based upon the plastic composition of the deck material.