You are missing everything! :)

Since DEC is not very liquid, and we can't manufacture DEC. As soon a price of DEC gets at peg (or even near peg), people can burn SPS to get DEC. SPS is very liquid and easy to get. At 1.02 above peg even company will burn SPS to get DEC to sell for credit. Lower transaction fee gives everyone more incentive to burn SPS, thereby lowering its supply.