Well, I questioned this at first too. But, Fran pointed out to me that we have come a long way in forensics. This is a NEW test.
Right. Because it was such a tiny, tiny amount. It is "touch" DNA - skin cells from a person touching something.
How could the perp have touched JUST the tights/panties and nothing else on Jonbenet's body or clothes.........or the weapon/garotte.........or the rest of the house?