Schizophrenia is a progressive disease in most cases. Starts with slightly weird behavior, and gradually progresses to full blown insanity. The earlier stages are often indistinguishable from other milder mental illnesses. And not all of these people see their doctors regularly, and not all of their symptoms are obvious all the time, so even if they do make it to monthly psych appointments, it's not necessarily going to be obvious in the standard 30 minute chat that the situation has changed and warrants a new diagnosis. Unfortunately, this can often mean a delay of several months, before a needed revision of diagnosis occurs.