Actually there are some physical differences in the brain which they are still researching.Speaking as a non-scientist ,it is to do with the chemicals which act in the brain between the synapses when you are using your memory and concentration.Those 'suffering' with ADD have a lack of this chemical and so they are easily distracted.In fact they often speak faster as sometimes their train of thought can go in the middle of a sentence.The chemicals that are commonly used in treatments try to correct this imbalance enabling kids (and adults too-you don't always grow out of it,but learn to compensate and deal with it better)to focus and calm down.Sometimes these pills can turn them into zombies,they are better at school but lose a creative spark.
You may be right squad in that they over-diagnose hyper or naughty kids with ADHD but once diagnosed very little is done about it.In most schools they just label them with it,put them into a useless special needs class once a week,and give them extra time in the exams.
Having had experience (and a bad one at that) in all this I could talk,argue and moan all day about it-I just wanted to encourage Lisa to have faith in her own thoughts and try not to get bogged down in all these tests.