I believe the charge has more to do with intent. Did he go after the kid intending to kill him, or did he go after him just intending to harass or even just to observe him, and the death was incidental yet caused by his actions. That would be the main difference in the varying degrees of murder vs manslaughter.
I may be totally off-base with this, but my thought was that the charge of "murder" in the Zimmerman/Martin case was perhaps a bit of overcompensation for the fact that initially the Police Dept. and Florida DA's office were ignoring the entire episode. It was almost a month later before anything was done to investigate the incident.