Coverage shadows. Those tips are pointing nearly straight down unless you roll the boom piping to point rearward. As the sprayer travels, mainly only the leading side of the plants get coverage, and the trailing side is left in the shadow with little to no coverage.

 You can roll the boom piping rearward to help some with the idea of allowing the solution to fall straight down, but then you will have the tops of the plants covered and not so much the sides.

 By using a tip setup that has both forward solution spray and rearward solution spray, you still have that double spray coverage that you mention side to side, but now it is 4 times covered and the trailing side has much reduced shadowing.

 Think of it like a fine powder lime application on a cloddy field. If you stand and look at the field from the same direction as the spreader travelled, you'll see perfect covered clods. But if you turn 180 and look at the field from the opposing direction of travel, you'll see very little white, looking like the application was almost not done accept in the tire tracks.