I Increased Retrieval From Top-5 to Top-20. My Answers Got Worse

The standard advice for improving RAG retrieval quality is: retrieve candidates, then filter down. Bigger pool, better reranker, better answers. I followed that advice in my RAG System. On PDFs, going from top-5 to top-20 made my RAGAS scores drop. The answers got worse, not better. Here's what actually happened and the experiment design that explained it.