Or that, like many mental conditions, others can mask or appear to be ADD/ADHD. There are plenty of individuals diagnosed with ADD, in which it manifests as a singular facet to an otherwise normal mind. Then there are plenty where it's a manifestation of combinatory factors from other mental illness. It is important to note though, that even as a singular facet, many of the resultant symptoms of ADD overlap with depression, anxiety and Bipolar Disorder (depending on the individual); despite the different effects and regions of the brain targeted. I would actually argue that this is one of the main culprits of its overdiagnosis in the US.
I would recommend reading into Russell Barkley's work. He's one of, if not the, preeminent experts on the Attention Deficit scope of disorders and studied their effects on the brain and really defined the modern idea of what ADD/ADHD are.