ADHD can manifest itself in very different ways. Everyone, adults and children alike, occasionally forget things, go through periods where they find it difficult to concentrate, and can even become hyperactive after having to sit too long in class or the office! Usually, ADHD is diagnosed when inactivity, hyperactivity and impulsivity happen in all of the person's life situations (like school, home, work, pleasure activities, play etc.)
However, I would guess that some of the obscure symptoms of ADHD are a result of these inattention or inability-to-focus symptoms that accompany those with ADHD. For example, a child who is unable to focus in class might have a hard time learning to read/do arithmetic/any other discipline; this inability to learn or remember might then be considered "symptoms" of the child's ADHD.
Remember that the symptoms of ADHD can be attributed to any number of things. Even before a formal diagnosis, which may not even be necessary, you can address each of the symptoms from which you/your child are suffering. For example, if one of the main problems is fidgeting, perhaps it you aren't moving enough in your daily routine. Do you exercise at least 5 times a week? Do you sit at a desk most of the day and never walk around? Or maybe an over-consumption of caffeine or sugar keeps you jittery. You might want to choose a less inflammatory diet and switch to some decaffeinated beverages. You might find that changing or tweaking some basic things in your life can help treat the symptoms from which you suffer. (This is not to say, of course, that all cases of ADHD can be cured through diet and exercsise, but in my personal opinion, MANY of them actually could be...)
Of course, obscure symptoms would not be used in a formal diagnosis of ADHD, nor should the presence of them necessarily be attributed to ADHD. After all, there are many other things that could cause a child to fall behind in school (if we're sticking with that same example). But obscure symptoms might well point to underlying causes that can then be treated.