Kosmo ... DI isn't a straight down shot. It's still angled out to the sides ... so it could be picking up signals bouncing off the sides of the fish. It's possible that the fish are not directly under the transducer, and the sonar sweep is catching these fish from an angle.
Just remember that the 2d sonar image in the left screen shot ... is, more or less, a straight down shot ... and therefore is interpreting the returning pings as "arches". You can't really compare the two, since they're different technologies ... so they render the return signals in totally different ways.
... cp