If you read here http://en.wikipedia.org/wiki/Mirtazapine its main action is antagonist and most antipsyhotic medications are antagonist. So could Mirtazapine be called antipsyhotic because it acts mainly as antagonist on receptors?
It does block some of the same serotonin receptors that atypical antipsychotics block, such as 5-HT2A & 5-HT2C. The 2A one probably contributes some antipsychotic effects, but I don't know if mirtazapine is useful in psychosis. I'd assume it's not much good as a standalone antipsychotic, since drug companies like to push for any indication they can get and most probably found it unsuccessful.