Some doctors like to describe ADD and ADHD as mental conditions, I like to refer them as other doctors do as neurological/behavioral conditions. There are many mental disorders such as Bi-polar disorder that can be pretty similar to ADHD (specially because of mood swings). A child may be hyper as a result of food allergies and not ADHD and when properly treated, the child behaves normally.
An abused child can often exhibit similar characteristics, acting out or sitting and staring and drifting off and not paying attention. Ditto for kids with very stressful home lives or kids who are homeless, or kids who do not understand the language they are being taught in. Kids suffering from Bipolar issues may also present with similar issues. In fact the reverse happens often in that kids are diagnosed with bipolar disease when it really is probably ADHD. It requires a knowledgeable doctor and specific testing to figure out the differences.