I think the hardest thing for people without ADD to process is the inconsistency. Some days are better than others, at least in my world. This is the best way I’ve ever heard it explained is this.
ADD affects the frontal cortex of the brain which is in charge of executive functions (directing the brain on what to do, how to act, what to say, what to focus on etc.).
In that respect, it’s like the conductor of an orchestra, telling the strings, brass etc. when to play. When the conductor doesn’t have the music or doesn’t conduct properly, the resultant sound is less than melodious and can at times be painful to listen to. However, when the conductor is at his best,timing is together and music is heard.
The inconsistency of this can lead people believe that the behaviors they see are willfulness, disobediance or non-compliance instead of a true brain miscommunication.
I use this analogy alot, I only wish I remembered who told this to me to give them credit.
I hope it resonated with someone. 
, M