Well, one factor is that I believe you’re comparing apples and oranges. Marksmanship usually is a measure of accuracy on a firing range, with all the conditions ideal: a stationary target, no stress (I’m sorry, even compared to shooting for the championship cup, with a lifetime stipend isn’t going to be as stressful as shooting at someone you think may have a gun, and is talking about using it on you, or your teammates.) and often no time limit.
Now, if you want to say that you think that police officers should be trained in ‘shoot houses’ which do a much better job of simulating the stresses and surprises of actual real-life firearms use than firing ranges can I’ll agree completely. But consider simply the human costs of the FBI’s Hogan’s Alley, and you’ll see why most local police agencies can’t really use that level of training for what is a very rare situation. FTM, it’s my impression that most FBI agents see training there pretty rarely, too.
This isn’t meant to excuse the number of rounds fired. It’s a partial explaination, however, for both the high number of total rounds fired, and the relatively poor on target numbers. I’d also like to point out that, for this incident, 20 rounds isn’t quite the horrible numbers it seems at first. With five officers involved, that’s an average of four rounds per officer, something that a semi-automatic pistol, or a revolver can put out in just a few seconds. (Now, if it comes out that one of the officers shot most of the rounds, and that he reloaded, I’m going to have a very different view of the gun handling skills of the officer in question.)
As for shooting to immobilize - my understanding is that such a tactic is pure Hollywood BS. If you’re going to shoot at someone, unless you’re the Lone Ranger, the expectation is that you’re going to probably kill him. You, and your target, may get lucky, but no one trains other than to consider the use of a fire arm as a deadly force.
I suspect that some of the gun and marksmanship knowledgeble Dopers could offer more concrete explainations, but this is I believe a tolerably accurate overview.