"Difficulty" is a measure of how competitively the coins are bid for in the primary market.

But Dash doesn't expose all of its supply to that bidding process. Only about 45% of it is. The rest is supplies to existing holders at zero price. Those are then sold in the secondary market where the seller will be at a profit at any price all the way to zero. That's what results in a decoupling (as far as that portion of the supply goes) between mined price ("difficulty") and market price.