It is, in legal parlance, a 'self-serving statement'. The legal system in the US operates under the assumption that nobody will lie to inculpate himself in a criminal matter but that most people will lie to excuplate themselves and avoid or lessen consequences of their actions.
In some jurisdictions a self-serving statement may not be admitted in court unless there are neutral corroborating witness statements or evidence such as a security video.
Maybe Zimmerman's statement is truthful...I don't know, you don't know and the legal system does not consider it evidence or proof.