I think there are so many labels stuck on kids these days that it's no wonder parents get concerned and confused.
When kids start school this is a huge change to their life - new environment, faces, friends and routine - it's no wonder that many of them are excited and have difficulty sitting still for five minutes to concentrate. Also like any healthy 6 year old, they have bags of energy and need to work this off somehow. So I agree with you rather than all these new fangled labels being stuck on kids, natural explanations need to be looked at first, especially as you say with your own wee girl who is obviously very intelligent and is probably just bored at times.
Certainly if a child is falling behind with schoolwork, isn't socialising properly and so on, then perhaps there is need for concern, but even here the explanation may just be in the personality and temprement of the child - and this might be normal for them - rather than anything more serious.
This was a really interesting question.