Look: let’s face facts. If it had been Avary 100%, don’t you think that QT would get in even more trouble (ie. all the people saying he’s just a glorified hack cuz he rips off all his favorite movies). If it had been Avary, I think more people would’ve heard of Killing Zoe instead of True Romance or Nat. Born Killers.
I think Avary helps him come up with the storylines, the work on those together, but Tarantino actually sits down and writes it.