There are many different types of mental illness. The categories are set by the Diagnostic Statistical Manual (DSM). This is the latest research in psychiatry all compiled into a very large book. The book is often updated in light of new research and labelled with a roman numeral.
The DSM lists not only all the different types of mental illness, but also the symptoms of these illnesses. The illnesses are certainly not all cookie cutter, but they follow the same patterns of symptoms.
Many times new psychiatrists will be confused when an illness does not look like the stereotype. This means that this psychiatrist should consult other experts. The doctor does not have a wide enough breadth of experience to properly diagnose the condition yet.
Many times the psychiatrist will make a judgement call but it is wrong. People can end up suing the hospital for malpractice. This often occurs when people who are treated die untimely deaths due to suicide.