Different doctors say different things because... in my opinion


1) Everyone perceives mental illness in a different manner. They can and often do agree on things when group discussion is happened, but often they won't have the time to do so. You can see, the existence of Dissociative Identity Disorder is debated still among psychologists.

2) Time. It takes years sometimes to diagnose specific disorders as stated in the DSM V. For example, they have to consider things like, "Were you on X drug for 6 months before the onset of your 24-month schizophrenia or psychosis?" and speaking of psychosis, the patient isn't usually the most reliable person to ask to.

3) They have different approaches and opinions.