I can tell you for sure that recycled paper comes with a whole host of non paper items before processing. It gets “de-trashed” in the recycling process. Imagine 6 foot tall heaps of staples,paper clips, pens, coins, metal bindings and other items. Some things are harder to separate than others, the gooey sticky stuff off of envelopes, stickers and tape are particularly hard since they tend to clump together and gunk things up. I imagine plastics are in the same boat.