This crossed my mind while I read through this thread:
When you mount a weapon you remove a lot of the differences between different types of firearms. Compare a pistol, an assault rifle and a rocket launcher (assume a heavy mount).
When hand-firing, they all have different stances. When mounted-firing you use the same stance for all three.
When hand-firing, they all have different recoil your body has to compensate for. When mounted-firing you don't have to worry about recoil.
When hand-firing, they all have different sights and aiming methods. When mounted-firing you are usually sighting them using the mount.
When hand-firing, they all are different shapes and weights. When mounted-firing you control the mount rather than the gun so differences don't matter.
There are minor differences in operation, but there are more similarities when firing a mounted weapon than differences. All of these cases also apply to lasers, the differences in stance, recoil, sights, and weight all cease to matter when you mount it.
That seems to be why the Gunnery skill works the way it does.