Regardless, I'm not overly concerned with the minutiae because in this particular case (State vs. Zimmerman), the self-defense claim is built on several conflicting reports from Zimmerman himself and it's quite obvious they are simply an attempt to justify the homicide. In this particular case against Zimmerman there is no justification for homicide and that is why I believe he will get manslaughter rather than murder.
Zimmerman's claim that Martin was on top of him beating him up is based on physical evidence (wounds to Z, no wounds to M) and eye witness account. There is absolutely no impartial evidence as to who started the physical alteration. Zimmerman's story of being attacked by Martin has never wavered, supported by the physical evidence, and the history of Zimmerman's interaction with the police when he was reluctant to approach other suspicious individuals in the past (BTW: he has been a concealed car permit holder for 4 years, so he was probably armed then too).