Well they give you 2 categories, extrovert and introvert, but neutral is a balance between the 2!
he who laughs the loudest and she who is always miserable, a real nasty person and the goody goody that's just as annoying. From one extreme to another, the total opposites. To be neutral is to be good but a little devilish, to laugh naturally instead of it being fake, standing up for yourself without being a bully, being funny without being a total clown, to show sadness without being the most depressing bore around, to care without wrapping in cotton wool. The list is endless, to me, that's normal, acceptable behaviour is neutral/balanced