Welp, I think that is due to many Police officers just have terrible aim.
I think the problem with the Self Defense positioning is the lack of witnesses and usually only one victim is living and one victim is dead and the dead victim, where evidence is not allowed to tarnish the dead victim, is the one presumed innocent rather than the person on trial. Unless it is unbelievably quite obvious of self defense, such as ya actually do have broken ribs, hints of concussions, or broken bones, and then ya kill the assaultor, then only then it is properly called self defense in the eyes of the jury.
According to the physical trainer who was teaching him how to fight, that is describing Zim as being a girly man, I think Zimmerman truely had a very low pain tolerance so, after fighting a while and all the obvious good hits the kid had on Zim, so then Zim was quick to shoot him since he could not take the pain anymore. Zimmerman should have allowed himself to gotten beaten up much more or even allowed the kid to grab the gun, for Zim to properly position himself as self defense.
If someone got out their dang house to approach the two using some common sense, maybe watching behind a car 20 feet away or something to safely watched or even intervened, then there would be a better witness so for Zim to claim self defense.
I think this is just how it is with self defense cases. Ultimately comes down to perceptions and emotion rather than following the law, just how it is with juries.
Now, I think folk can demonstrate so to reasonably corrupt the jury by stating how that Zim showed wreckless intent by being so quick to shoot the kid too soon into the fight.