I'm just speculating here....do we know if these clothes were recently washed? If so, then the DNA means something. If not, JBR could have changed clothes elsewhere and someone picked them up to put with the rest of her belongings.
My 4yo niece has changed clothes at our house, and both my husband and I have picked up her clothes (left on our bathroom floor) to put with her backpack. I never checked, just grabbed them, but they could have been inside out, therefore exposing the crotch and insided of her pants.
I know they have checked everyone who is everyone's DNA, but it could have been someone who was a guest in someone else's house.
"Hey, could you pick that up and toss it over here so I can put it with JBR's stuff?"
It's a stretch maybe, but when you have little kids and they are changing clothes. Heck, my 13yo still leaves her undies on the bathroom floor after she showers! Believe me, I've explained until I'm blue in the face that no one really wants to see her undies!