I think it is more how the weapons are grouped.
x1 cannon x2 cannon "3 cannon only two damage groups"
Or just x2 cannon. "2 cannon 2 damage groups"
Thats why is you get one cannon hit it automaticly takes the other one out aswell, while if you only have 2 to start with, you would see the damage effect as it should.
Therefor its a problem more likely with the la7 needing one more x20mm to be "separate" from the other two cannon packages. [hit groups or whatever]