Psychiatric diagnostic labels are based on symptoms, and a lot of the different conditions have overlapping symptoms, or have the symptoms of other conditions as a sort of secondary effect.

In other words, if you have three problems, you'll get diagnosed with like ten things! (slight exaggeration)

Two examples from me:

I have some kind of cyclical depressive disorder wherein I tend to be sad all the time for no reason but the severity fluctuates a lot. There isn't a diagnostic label for exactly that, so even though it's just one problem, my psychiatrist had to give me gave me two simultaneous diagnoses (dysthymia and recurrent major depressive disorder).

The abovereferenced depressive disorder is perfectly treated with medication, but I also have ADHD, and that has been a much harder problem to solve. So, I have an untreated neurological disorder which has a severe impact on my ability to function. Naturally, I'm sad about that. The psychiatrist sees how sad I am and thinks the depression treatment is not working well enough! I'm pretty sure he would have diagnosed me with depression even if I didn't have it.