I'm not Grinity, but I can answer this, at least partially.
Some kids are shy or passive and don't like to ask for things or push adults, instead waiting for things to be offered, so while they may be capable of of learning very quickly, and may have absorbed a great deal, they may not demonstrate it spontaneously, and may not ask for more.
Some kids like to fit in and are wary of behaving in ways that make them stand out from their peers, and may not show what they know, even to parents, because it is important to them to be seen as fitting in and not drawing attention to their differences.
Some kids may have "behavior problems" or "attitude problems" that are actually boredom problems, but because they appear to be uninterested in "learning" (because they mastered the material on their own long before) and perform poorly in class (because they are bored and/or insulted by being asked to do such simple material), giftedness is not suspected. A child whose response to this kind of boredom and frustration is to mouth off or be a class clown is often a gifted child but is often not identified as such.
Hidden learning disabilities can also complicate the identification of gifted children by making it more difficult for them to demonstrate what they are truly capable of.
These are just a few of the ways that personality and other issues can interfere with the easy identification of giftedness.
