Think about elderly people versus teenagers walking around in the summertime.
We often see elderly people wearing winter coats (because they are elderly and have Yang Deficiency), while the teenagers (who are full of Yang) are in shorts, sandals, and t-shirts.
Now, if I catch a Wind-Cold, it is VERY HARD to get warm. If I put a winter coat on myself, I still might not warm up.
This is the diagnostic difference.
Great question.