I really appreciate you asking on this because I realize some posters may view this theory as quite far-fetched. (And maybe it is way off.)
He had two victims. He did not always treat these victims the same. Initially, as far as we know, though, he did. "Guys," down the hill. Both young girls are "guys." To him, so far, they are both to him the same. But then, things change. Libby's injuries are different. She has multiple injuries, Abby has the single cut mark. The clothes are different. Libby has been left out there with no clothes, Abby is almost fully clothed. In fact, Abby has clothing original to her for many items and what she doesn't have, she has in some cases been dressed in Libby's clothing. The placement of the bodies is somewhat different. Libby is near that tree, arm upright. Abby's arms are kind of folded inwards to her chest. And Libby has the phone and shoe beneath her.
The bullet is between the victims. MOO, he wanted separation between these victims, and my guess is he killed them at least in part to gain that in his own head. The bullet at home was, I believe, perhaps his only souvenir from that CS. Having been "interrupted," he was probably not in much of a mood for souvenirs (although you never know). But the bullet was special, and the bullet was an unknown: it wasn't something someone would "miss" because the viewer would have to know it was introduced at the scene to begin with. It is a mystery to which only he knows the answer. The bullets for him are incredible power. (Definitely) MOO, in his own sick head, he has some strong (fantasy) connection to one of these girls and really, really resented the other girl, the dear best friend.
He is a horror.