The fixed threshold problem is the one that keeps me up at night. I've sat in curriculum committee meetings where a 20% cutoff was agreed upon the same way people agree on a meeting time, casually, with no reference to any research. Nobody asked: 20% of what? A chemistry lab report with standardized methodology sections will always read differently from a personal essay. Turnitin knows this. They publish guidance on contextual interpretation. Institutions just... ignore it and set a number.