Two other factors: time and the defender's capability.
Time: you build in no assumption of the dispersal of assailants nor moving targets after the first discharge.
Capability: civillian shooters, last I saw, were seven (7) times more likely to hit their target than police. Cops are notoriously bad shooters (I can tell you this is quite true, and they are actually very dangerous) -- essentially failures at a core responsibility of their duty.
There are many examples but one famous one, especially to the Left, is that of Amadou Diallou (sp?). Four of the elite NYPD Street Crimes Unit fired 41 times (two emptying 16 rounds each) and he was hit 19 times at the top of the stairs into an apartment building from the bottom of them. 11 times in the legs. "Contagious shooting." There are others even MORE lopsided.