One,
You'll see this happen if,a) the target is out past 500 yards or so,the default convergence range.
b) the target is inside the convergence range.
Lusche has posted a pic of the defencive guns on bombers and it clearly shows why some guns arent firing at the exact spot your aiming depending on the range of the target.
This can be useful,you can fly just above convergence range,say 800yds,and strafe down plenty of town buildings because of the dispersion pattern.