Because Zimmerman is the interested party, d’uh! Electric chair or walk free, do I have to spell it out? Apparently so. Not a word he says is believable.
You can believe what he said and was recorded on the 911 tape, which was: "f'n coons".
The rest he said was a lie to cover his desire and intent to shoot Martin for his own racist pleasure. He already told the cops that Martin had his hand in his waistband and had something in his hand, so he never would have let Martin attack him w/o having pulled his gun out first -if he was actually in fear of him as an attacker. He wasn't in fear of Martin though, because he was armed and ready to shoot him after he egged Martin on to attack him him with a flurry of racial slurs. That attack was Zimmerman's planned cover of self defense for the murder.