Why do people without balding also have miniaturized hair or vellus hairs in their scalp? You say that 10 percent of miniaturization is normal, whats going on with this?
Vellus hairs are part of the normal follicular unit. They usually grow very short and are finer. They also do not have a sebaceous gland associated with it supplying sebum to the skin.
I can not answer your why question, as this is just what we have observed and reflects our observational science.