I think there are 2 possibilities.....Wilbur is close with the affect of the steep slope.......In the 2d return bottom indication is very thick and the fish can get lost in this thick bottom return.....The 2D is seeing a greater range of depth returns because it is getting returns from the front of the cone as well as the back of the cone on the drop.
The DI isn't affected as much because it is very thin front to back and won't be affected as much by the slope.

The other possibility is that the fish is further off to the side and not in the 2D cone......The Di is wider than the 2D

I can visualize this, but not sure if I stated it coherently.