It's possible to have been through childhood abuse and have no disorder. That doesn't mean a person is unaffected, or that their abuse was somehow lesser than another persons (an assumption too often made). It can sometimes mean that their coping mechanisms have enabled them to cope on the outside.
It's also worth noting that diagnosis is wholly dependent on visiting a doctor and what that doctor diagnoses. If you go to different doctors, or go to doctors at different times in your life, it is more likely that you will end up with a list of different diagnoses. Likewise, if you never seek help (or get sectioned) then whatever mental health issues you have won't be diagnosed.
Sadly, for some people who have always been abused, no one cares enough to ever say that they might be helped. The diagnosis then has terms like 'lazy', 'delinquent', 'alcoholic', 'druggie', 'good for nothing', 'tramp', 'crazy' etc.
I'm sure that kas-can-fly was only joking, but I don't think people with only 1 diagnosis are 'luckier' than people with multiple diagnoses. They are all just a reference for symptoms. In my mind, there's too much emphasis put on diagnostic labels, and not enough on the traumatic experiences of the person.