Broccoli's point a few posts back sprung something into my mind.
Maybe LV didn't use his own wand, and the wand he used, say James or Lily's was infused with good magic, so when a wielder cast a particularly evil spell, say... Avada Kedavra, it would rebound upon it's holder.
Why wouldn't he be using his own wand though?
I wouldn't question out Lily disarming LV, she was a powerful witch. Unfortunately not powerful enough, but powerful.
Or maybe Priori Incantatem doesn't show every spell you've cast in reverse order, it just shows all the people that have been killed by the wand. Maybe JK got it wrong?
No, that cannot be there must be a REASON!