There is some natural variation in hair diameter
In non-male pattern baldness people, there is a 20-25% variation in diameter of the hairs in any given area. That is, all the hairs are not equally thick. In the human body, nothing is 100% perfect, symmetrical, or consistent, and hair is no exception. If you look closely at a non-male pattern baldness person's hair (try to look at some of the hairs which are standing up above the rest), you will see that not all the hairs are exactly the same diameter. Approximately 18 out of 20 will be the "average" hairs, there will be one "thicker than average" hair and one "thinner than average" hair. Although I have fine hair, a few of my (thickest) hairs would be equal to the "average" hairs of a person with very thick hair. So a few random slightly thinner hairs does not necessarily mean anything. If in general, you shed normal thick hairs of a decent length and one or two slightly thinner but just as long hairs, its NORMAL. What you should worry about is when many of the hairs are thinner i.e. a significant reduction in the average diameter. They (the thinner hairs) will also be shorter and have tapered ends (indicating shorter anagen cycles) And remember, since the thinner hairs affected by DHT also have longer telogen phases, they are more likely to be shed. But don't worry.... finasteride or minoxidil will be able to thicken up the hairs for most people.
And remember, the nape of the neck always has some vellus hair at its lowest portion. Its a "transition" zone, so to speak. Even the frontal edge of the temples (NOT the hairline above the temples, but the actual temples) has some finer, vellus-like hair.
Cheers,
Arjun