Okay, I'm surely not the sharpest knife in the drawer, but I'll give this a shot. I'm just looking at this from the most obvious perspective.

The leading and trailing edge is where the beam is hitting the fish from an angle as the fish enters or departs the cone and the thicker part of the arch is where the beam must pass through(or is blocked) on the fish while the transducer is directly over the fish which in turn gives it the thicker display in the middle. That would show the beam hitting a fish and giving it the arch shape and steadily becoming thicker upon approach and thinning as the fish is moved towards the trailing edge of the cone.