The 3-5 bullets missing would be, well, missing, and therefore, it isn't really a 100% aim.
If you lock with SMGs, it's always going to hit, but if the bullets miss, that means 100% aim, really isn't 100% aim.
Marksmanship, is how many times your shots hit. 100%, would mean all your shots hit, as in NO bullets miss.
There's no difference testing with cannons or ranged series. If one misses, then all of them miss.