In dead 6 shots, due to the scatter, a lot of the bullets are very near misses. Not all hits produce a hit sprite so it is hard to see since the max number of sprites is already showing. When you hit a larger surface all bullets will hit even with some scatter.
However, the most important aspect is how planes die in AH: you need to break a wing/tail, set fire to the engine or kill the pilot. When you shoot from dead 6 the parts that are most likely to take the damage are control surfaces: flaps, elevators, ailerons, rudder. They will soak up the damage and even if destroyed, the plane is still flying. You are much less likely to hit any of the critical components mentioned above. In top plane shots everything critical is exposed. If you fly the twin engine fighters, one of the more useful things you will learn is how much more survivable the plane is if you roll the plane to take the hits on the side instead of the very large top.